BG3 Underdark Guide: Best Route, Hidden Items, and Why You Shouldn’t Skip It for the Mountain Pass

Verified for BG3 Patch 8 (the final patch). All mechanics, item locations, and fight strategies reflect the current game state.

Act 1 puts two routes south on the map at roughly the same time: the Underdark and the Mountain Pass. Most first-time players take the Mountain Pass because it looks like a direct road. That’s the wrong call.

The Underdark is where Act 1’s best loot lives — two Very Rare crafted items from the Adamantine Forge, a party-wide buff weapon in Phalar Aluve, and Mourning Frost for cold casters — all lost if you skip it or rush through without the right sequence. This guide covers the route decision with specific reasons, the optimal zone order that avoids permanently missing content, every key item and how to get it, and a three-step strategy to beat the toughest Act 1 boss at level 5.

Quick Start Checklist

1. Clear the Goblin Camp first — be at least level 4–5 before entering the Underdark
2. Enter via the Defiled Temple in the Shattered Sanctum (no puzzle required, most natural from Act 1 progression)
3. Make the Myconid Colony your first stop — the only safe hub with traders and quest-givers
4. Follow this order: Selûnite Outpost → Dread Hollow → Arcane Tower → Myconid Colony → Festering Cove → Grymforge
5. Do the Mountain Pass after the Underdark — both routes are available until you enter the Shadow-Cursed Lands

Underdark vs Mountain Pass: Which to Take First

Neither route locks out the other. You can — and should — complete both before triggering Act 2. The Act 2 trigger is entering the Shadow-Cursed Lands, which happens at the end of either route. Once you cross that threshold, all unfinished Act 1 quests auto-fail in both areas.

Order matters for one specific reason: the Mountain Pass has a visible elevator pop-up warning before its point of no return. The Underdark has no equivalent brake. Players who start with the Mountain Pass typically get drawn into Act 2 before returning, permanently losing access to the Adamantine Forge and every time-sensitive Underdark quest.

The Mountain Pass is genuinely worth doing for the Blood of Lathander (a legendary mace inside Rosymorn Monastery) and Lae’zel’s companion arc. Neither becomes missable by going Underdark first. The Underdark, by contrast, contains time-sensitive quests, a permanently missable NPC (Philomeen and her runepowder barrel), and the only Adamantine Forge in the game. The risk asymmetry is clear: the Underdark has more to lose from skipping.

One narrative note worth knowing: the Underdark exit deposits you at the Last Light Inn in Act 2 — the Harpers’ faction hub and the “good guys first” introduction. The Mountain Pass route leads toward Moonrise Towers first. Both paths converge, but arriving at Last Light before Moonrise Towers gives clearer context for Act 2’s central conflict. It’s not a mechanical difference, but players who find Act 2 narratively confusing often went Mountain Pass first.

How to Reach the Underdark (4 Entry Points)

Four separate paths lead into the Underdark. Which one you use determines your starting location inside the zone.

• Defiled Temple (Goblin Camp) — Recommended: After clearing the Shattered Sanctum, find the lever on the eastern room wall — you do not need to complete the moon puzzle. Drops you at the Selûnite Outpost waypoint, the best starting location for the route below.
• Zhentarim Hideout: Elevator behind Waukeen’s Rest. Navigate the hidden hideout (northwest map). Drops you near the Storehouse — useful only if you’re approaching from the north side of the map.
• Whispering Depths: Jump into the spider cave pit northeast of the Emerald Grove. Requires Feather Fall or a flying character — the fall is lethal without it. Lands near the Selûnite Outpost like the Defiled Temple route.
• Overgrown Tunnel (Mushroom Circle): Portal in the Pale Elf’s cave northwest of the Grove. Requires a Whispering Mask from the same location. Functional but slower to access than the Goblin Camp route.

If you cleared the Goblin Camp via the Shattered Sanctum, use the Defiled Temple entrance. The transition is natural, and it drops you directly at the most efficient starting point for everything below.

The Optimal Route Through the Underdark

The Underdark spans seven distinct zones across a wide map, and the content in each connects in non-obvious ways. Miss the Duergar Intruder quest in the Myconid Colony and you permanently lose one of Mourning Frost’s three components. Rush to free Nere in Grymforge without finding Philomeen first and her runepowder barrel disappears forever. The sequence below avoids both.

Step 1: Selûnite Outpost — Phalar Aluve and the First Mourning Frost Piece

Before moving anywhere, grab the Selûnite Outpost waypoint and head west. Phalar Aluve is embedded in stone at X: 116 Y: −192 — a rare +1 longsword with a Sing ability that grants all nearby allies a 1d4 bonus to attack rolls and spell save DCs within 9 metres. Pull it with a DC 15 Strength check, a DC 15 Religion check (Lolth-Sworn Clerics get advantage), or by dealing damage to yourself and spilling blood on the blade.

Also in this area: Dhourn, a hostile mage northwest of the outpost. He carries the Icy Helve — the first of three components needed to assemble Mourning Frost. Kill him and loot it before moving southwest.

Step 2: Dread Hollow — Filro and the Noblestalk

Head southwest to Dread Hollow. Two pickups here:

• Filro the Forgotten: A hostile wizard near the Sussur Tree. He carries the Icy Crystal (Mourning Frost component two). He aggroes on approach — kill him and loot it. Avoid fighting in the Bibberbang mushroom field nearby; the mushrooms detonate in chain reactions on contact with fire.
• Noblestalk mushroom: Found near Baelen Bonecloak in the Bibberbang field. Used in camp to restore memories for Shadowheart or the Dark Urge origin. Not required, but permanently missable and useful for completionists.

Step 3: Arcane Tower — Three Items Worth the Full Detour

Continue southwest to the Arcane Tower in the far corner of the Underdark. The entrance is guarded by arcane turrets. Pick a Sussur Bloom from the ground near the Sussur Tree (same area as Dread Hollow) and insert it into the ground-floor generator — this disables the turrets and restores tower power.

Do not leave this tower without:

• Club of Hill Giant Strength (basement): Smash the Stool of Hill Giant Strength to receive this club. Equipping it sets the wielder’s Strength to 19. For any STR-based martial at level 4–6 — Fighter, Paladin, Barbarian without high base STR — there is nothing better available until Grymforge crafting or Act 2 gear.
• Guiding Light ring (fourth floor): Find the Threadbare Book on an upper floor and read it. This provides the correct Bernard dialogue line: “How can I show myself, my darkest me?” Use this phrase with the automaton Bernard on the roof — he gives you the Guiding Light ring, which also unlocks the basement trapdoor. The ring provides a free Light cantrip and allows seeing in magical darkness without the Darkvision racial feature.
• Mystra’s Grace boots (third floor): Allows casting Misty Step even while falling. Useful for spellcasters navigating the Underdark’s many drop-hazards, and strong in Act 2 platforming sections.

Step 4: Myconid Colony — Quest Hub and Mourning Frost’s Critical Third Component

Head north to the Myconid Colony in Ebonlake Grotto. This is the Underdark’s only safe hub: traders Blurg and Derryth Bonecloak sell unique scrolls and crafting materials, and the illithid Omeluum is here to advance the parasite investigation quest.

The dependency most guides miss: before you can access the area containing Xargrim’s corpse — which holds the Icy Metal, the third Mourning Frost component — you must complete the Defeat the Duergar Intruders quest. Skipping this quest while collecting the other two components leaves Mourning Frost permanently unfinished. Complete Defeat the Duergar Intruders, then loot Xargrim’s corpse for the Icy Metal.

With all three components in one character’s inventory, right-click any one of them to find the combine option. Mourning Frost is a Very Rare +1 quarterstaff that deals 1d8+1 bludgeoning plus 1d4 cold damage (two-handed), adds the Heart of Ice passive (bonus cold damage on hits), and applies the Chilled condition to enemies hit by cold spells via the Insidious Cold passive. For cold-focused spellcasters, this staff outperforms every Act 1 and most Act 2 options until endgame gear. If you’re running a cold build, check our BG3 sorcerer build guide for how to build around it.

Step 5: Festering Cove and Final Underdark Cleanup

The Festering Cove is a hidden area accessible from the eastern Underdark. Kuo-toa here worship a creature named BOOOAL as a deity. A DC 20 Investigation check or DC 15 Persuasion check exposes it as a fraud — the resolution grants either a new ally faction or access to the Sickle of BOOOAL. The Slippery Chain Shirt from the gilded chest is the better pickup for DEX builds who haven’t found better medium armor elsewhere.

After Festering Cove, you’ve covered all the time-sensitive and permanently missable Underdark content. Take the skiff from the Decrepit Village beach (X: −7 Y: −210) to Grymforge.

Don’t Leave the Underdark Without These Items

Mourning Frost: Best Cold Caster Weapon in Act 1

The assembly chain’s dependency structure is what makes Mourning Frost genuinely missable — most guides list the three components but don’t mention that the Icy Metal requires completing Defeat the Duergar Intruders in the Myconid Colony before the corpse area is accessible. If you collect the other two pieces and skip that quest, the staff cannot be finished.

Why it matters: Heart of Ice adds bonus cold damage to every attack, and Insidious Cold applies the Chilled condition (reduces movement speed and defence) via cold spells. Cold-damage sorcerers and wizards deal meaningfully higher damage with this staff than with anything else available in Acts 1 through mid-Act 2. The 1d4 cold bonus on every hit also makes it a functional martial weapon for Druids who want to avoid upcasting slots.

Phalar Aluve: The Best Act 1 Support Weapon

The Sing ability is Phalar Aluve’s reason to exist. Activating it grants all party members within 9 metres a 1d4 bonus to both attack rolls and spell save DCs. At level 4–6, a +1–3 average bonus to every party member’s attacks is more total damage contribution than almost any weapon action on a single character. It’s worth equipping even on a character who deals low weapon damage, purely for the aura.

Phalar Aluve also has the Finesse property, which is unusual for a longsword — it can use DEX instead of STR for attack and damage rolls. This makes it the best Act 1 weapon for DEX-based martials like Rogues and DEX Fighters who would otherwise be stuck with shortswords or rapiers for scaling.

Club of Hill Giant Strength: Transformative for STR Builds

Setting Strength to 19 at level 4–6 changes melee combat. Before better gear appears in Act 2, this effectively adds +3 to +4 to your frontliner’s attack and damage rolls. The tradeoff — the weapon itself is a Club with a 1d4 damage die — is real but manageable: most players equip it in the main hand for the STR floor and use a higher-damage weapon via a different attack. For Fighters using Extra Attack, the club’s low damage die shows; for Paladins who want to ensure Divine Smite hits land reliably, the accuracy bonus is worth it.

Grymforge: Prep, Craft, and Beat Grym

Before You Reach the Forge

Grymforge is content-dense with real consequences for order-of-operations mistakes. These five points prevent the most common permanent misses:

• Find Philomeen before approaching the cave-in rubble. She holds the runepowder needed to clear the rubble. If you interact with the cave-in area and free Nere without contacting Philomeen first, she disappears permanently and takes the runepowder barrel with her. No other source for it exists.
• Decide your two adamantine items before reaching the forge platform. The six moulds are scattered across Grymforge but some are in areas accessible only before the Grym fight begins. Know what you want before pulling the forge lever.
• Free the enslaved gnomes early. The cave-in rescue quest is timed once you approach the rubble — it fails if you delay too long after triggering it.
• Paladins: do not side with Nere. Allowing him to kill gnomes breaks your oath. If an Oathbreaker run is your goal, our Oathbreaker build guide covers the full conversion and what you gain from it.
• Collect all six forge moulds before activating the forge sequence. They are spread across the Abandoned Refuge and Merregon’s Hideout sections of Grymforge.

For a full list of time-sensitive quests across all of Act 1, see our BG3 time-sensitive quests guide.

Adamantine Forge: Which Two Items to Craft

Only two pieces of mithral ore exist in the entire game — one near the Abandoned Refuge cliffs (X: −646 Y: 253) and one in the library area east of the main Grymforge bridge. You get exactly two adamantine crafts. The mithral ore locations require defeating the Magma Mephits in the library area before the second vein is accessible.

Adamantine gear shares one property regardless of type: attacks ignore resistances. This makes it unusually valuable in Act 2 where many enemy types resist or are immune to specific damage types.

If you have two STR-based melee fighters, Splint Mail and the Scimitar or Mace is the standard optimal pair. If only one character uses heavy armor, Splint Mail plus Scale Mail for a medium armor user covers both frontline characters through Act 2.

Beating Grym: A Mechanic Check, Not a Gear Check

Grym is a level-10 construct with 300 HP on Balanced difficulty, and he is immune to every damage type in the game — all physical types, all elemental damage, psychic, radiant, and force — until you apply the Superheated condition. There is no conventional approach to this fight. Players who attempt it without understanding the forge mechanics wipe repeatedly, conclude they are underleveled, and return at level 8. They are not underleveled. They are missing the mechanic.

The three-step strategy:

1. Pull the lava valve to flood the forge platform with lava. This applies the Superheated condition to Grym, switching his vulnerability profile to bludgeoning.
2. Lure him onto the central anvil. Minor Illusion works reliably because his Intelligence is low — he will move toward the auditory illusion without requiring line-of-sight manipulation.
3. Pull the forge hammer lever. It deals 12d8 bludgeoning damage, doubled for his active bludgeoning vulnerability. The hammer deals an average of 54 bludgeoning damage (12d8), doubled to around 108 per application against a Superheated Grym. With 300 HP on Balanced, two to three hammer applications clear the fight — your party need not engage directly in conventional combat.

You do not need to be level 8–10 for this fight. That recommendation applies only if you attempt conventional damage output, which is not viable before Superheating. The forge hammer strategy works at level 4–5 with no preparation beyond knowing the sequence.

Honour Mode note: Grym gains Adamantine Reverberation as a legendary action — 4d8 thunder damage and temporary HP after being hit. On Honour Mode, the forge hammer strategy is still the solution; you may need two full applications rather than one. Do not attempt sustained conventional combat on Honour Mode. On Balanced and Tactician, one lava application followed by a hammer hit ends the fight cleanly.

Frequently Asked Questions

Can I do both the Underdark and Mountain Pass in one playthrough?

Yes — both areas remain available until you enter the Shadow-Cursed Lands, which is the hard Act 2 trigger. Entering the Shadow-Cursed Lands auto-fails incomplete Act 1 quests in both areas. The correct sequence is: Underdark → Mountain Pass → Shadow-Cursed Lands. Check our BG3 time-sensitive quests guide for the full Act 1 missable content list.

What level should I be for Grymforge?

Level 4–6 is enough if you use the forge hammer strategy described above. The level 8–10 recommendation that appears in some guides assumes you’re trying to fight Grym through conventional damage, which is intentionally impossible until Superheating. The forge mechanic removes the level requirement — knowing the three steps is the only prerequisite.

What content is permanently missable in the Underdark?

Two locked items: Philomeen’s runepowder vial (disappears if you free Nere without finding her first) and Mourning Frost (requires the Duergar Intruder quest before the Icy Metal component is accessible). The gnome rescue quest also fails if you delay after triggering the cave-in sequence. None of these have in-game substitutes.

Does going through the Underdark affect Act 2?

Not mechanically — both routes access the same Act 2 zones and neither locks any endings. The Underdark exit leads to the Last Light Inn first; Mountain Pass leads toward Moonrise Towers first. Meeting the Harpers before Moonrise Towers typically makes the Act 2 faction dynamic clearer on a first playthrough.

Does Underdark-first delay Lae’zel’s story?

Slightly, but not harmfully. Her Githyanki Crèche questline in the Mountain Pass remains fully available after the Underdark. The only consequence is a one-area delay on her personal arc. For Lae’zel-focused playthroughs, this is a minor tradeoff against the loot advantage. Our BG3 best companions guide covers her full story path and the Crèche decision implications.

Sources

• bg3.wiki — Underdark
• bg3.wiki — Grymforge
• bg3.wiki — Grym
• bg3.wiki — Phalar Aluve
• bg3.wiki — Arcane Tower
• bg3.wiki — Mourning Frost
• bg3.wiki — Travel through the Mountain Pass

Michael R.

I've been playing video games for over 20 years, spanning everything from early PC titles to modern open-world games. I started Switchblade Gaming to publish the kind of accurate, well-researched guides I always wanted to find — built on primary sources, tested in-game, and kept up to date after patches. I currently focus on Minecraft and Pokémon GO.