BG3 Underdark Guide: Complete Walkthrough, Secrets & Best Items for Beginners
Complete BG3 Underdark guide for new players. Every entrance, the Myconid Colony, Arcane Tower, Grymforge, the Adamantine Forge, and the Grym boss fight — step by step with maps.
Underdark Overview
The Underdark is a massive underground region beneath Act 1. It’s entirely optional but contains some of the best loot, XP, and crafting opportunities in the game.
- Level range: 4-6 (enter at level 4-5)
- Major locations: Myconid Colony, Sussur Tree, Arcane Tower, Grymforge
- Bosses: Bulette, Spectator, Duergar, Grym
- Landmark loot: Phalar Aluve, Staff of Arcane Blessing, Adamantine gear
Part 1: The Myconid Colony
Your safe haven in the Underdark. The myconids (mushroom people) are friendly unless you attack them.
Key NPCs
| NPC | Purpose |
|---|---|
| Sovereign Spaw | Colony leader. Offers quest: kill duergar across the lake |
| Blurg | Hobgoblin merchant. Sells Melf’s First Staff (+1 spell attack AND +1 spell DC) |
| Omeluum | Friendly mind flayer. Offers help with your tadpole |
| Derryth Bonecloak | Dwarf searching for her husband. Starts the Noblestalk quest |
| Thulla | Poisoned gnome. Gives Boots of Speed if cured |
Duergar Beach Fight
Sovereign Spaw asks you to eliminate the duergar invaders on the beach across the lake. The duergar are led by Gekh Coal. Fight them, or use Speak with Dead on the dead myconid nearby to find an alternate resolution.
Part 2: The Sussur Tree
West of the Myconid Colony. A massive glowing tree surrounded by Hook Horrors and the Bulette (a burrowing monster that erupts from the ground).
Sussur Bark: Harvest from the base of the tree. Bring it to the Blighted Village forge to create a Sussur weapon (+1, silences targets on hit). This is one of the best weapons in Act 1.
Bulette Strategy: The Bulette burrows and erupts, dealing AoE damage. Spread out your party so it can’t hit multiple characters. Use the high-ground rock formations. It drops Bulette Scales that can be crafted into armor.
Part 3: The Arcane Tower
Southwest of the Underdark. A multi-floor wizard tower with unique loot.
Ground Floor: Turret Defense
The tower is guarded by Arcane Turrets that shoot force damage. They’re nearly immune to damage — but they’re vulnerable to lightning damage. Cast Witch Bolt or throw lightning arrows. Or, find Sussur Blooms (glowing flowers at the Sussur Tree) and throw them near turrets — Sussur Blooms create an anti-magic field.
Elevator Puzzle
The elevator requires power. Find the combination on a note in the tower: give a Sussur Bloom to the generator on the ground floor.
Key Loot by Floor
| Floor | Key Items |
|---|---|
| Ground | Sussur Bloom generator |
| Level 1 | Staff of Arcane Blessing — Bless gives 2d4 instead of 1d4. Invisible chest in the basement. |
| Level 2 | Poems and notes for Bernard puzzle |
| Level 3 | Bernard (construct boss). Answer: “The sun is warm, the breeze is light” |
| Level 4 (top) | Unique ring, balcony with a view |
Part 4: The Petrified Drow & Spectator
South of the Arcane Tower. A group of petrified drow surround a glowing crystal. When you approach, a Spectator (lesser Beholder) breaks free.
Spectator Strategy:
- The drow un-petrify and are hostile. Focus the Spectator first — it has dangerous eye rays.
- Spread out your party (eye rays are single-target)
- Use Blindness spell (Spectators have average CON saves)
- Dror Ragzlin drops can include Spectator Eyes — keep them
Part 5: Grymforge (The Adamantine Forge)
Accessed by boat from the Underdark beach (near the duergar camp). Grymforge is a massive duergar fortress and forge complex.
True Soul Nere
Nere is trapped behind a cave-in. This quest is time-sensitive — if you long rest too many times after arriving, Nere dies and you lose his quest rewards. Resolve the cave-in quickly:
- Use smokepowder bombs (from duergar or find them) to blow the rocks
- Or use Force damage spells
- Free Nere, then decide: side with him, side with the rebelling duergar, or kill everyone
Grym — The Adamantine Golem Boss
The forge’s guardian. Grym is a massive construct with 300+ HP and near-immunity to normal damage — unless you use the forge hammer.
Grym Strategy (The Easy Way):
- Grym is Superheated when standing in lava. Lure him into the lava pool.
- While Superheated, he takes normal damage. Hit him to build threat — he attacks the last character who damaged him.
- Lure Grym to the center platform (the circular hammer area)
- Use a ranged character standing near the forge lever to pull it
- The hammer drops, dealing 150+ damage to Grym
- Repeat 2-3 times
Adamantine Forge Crafting
After defeating Grym, use the forge to create legendary Adamantine gear:
| Item | Type | Effect |
|---|---|---|
| Adamantine Splint Armor | Heavy Armor | 18 AC, attackers can’t land crits, Reeling debuff on hit |
| Adamantine Scale Mail | Medium Armor | 16 AC, same anti-crit effect |
| Adamantine Shield | Shield | Attackers become Reeling when they miss you |
| Adamantine Longsword | Longsword | +1, ignores slashing resistance, crits on objects |
| Adamantine Scimitar | Scimitar | +1, same ignore-resistance effect |
| Adamantine Mace | Mace | +1, same effects |
Recommendation: Craft the Adamantine Splint Armor (if you have a heavy armor user) or the Shield. Two mithral ore veins exist in Grymforge — use one for armor and one for the shield.
Mithral Ore Locations:
- The main forge area (guarded by magma mephits)
- Near the waypoint, on a ledge (jump or Misty Step)
Frequently Asked Questions
Should I do the Underdark or Mountain Pass first?
Do both — they're not mutually exclusive. The Underdark is generally harder (level 4-6 content) but has better loot. The Mountain Pass is easier (level 4-5) but has critical Lae'zel story content. Recommendation: Mountain Pass first (level 4-5), then Underdark (level 5-6), then proceed to Act 2.
How do I reach the Underdark?
Four ways: (1) Jump down the Whispering Depths well in the Blighted Village using Feather Fall, (2) Take the elevator from the Zhentarim Hideout under Waukeen's Rest, (3) Solve the moon puzzle in Priestess Gut's chambers in the Goblin Camp, (4) Mushroom circle in Auntie Ethel's lair.