Baldur's Gate 3 Underdark: Complete Exploration Guide to Adamantine Gear & Hidden Areas

This article is based on information as of June 14, 2026. Baldur's Gate 3 is not a live service game and does not adopt a seasonal model. The latest major update was "Patch 8" (released April 15, 2025), and Hotfixes have been continuously released since then. The latest Hotfix is "Hotfix #36" (released March 26, 2026).

Your Act 1 adventure isn't confined to the surface. Deep below lies the Underdark, a mysterious and perilous world awaiting your exploration. While powerful enemies roam, this area is essential for strengthening your party, harboring numerous unique items and equipment valuable even in the late game. This article provides a comprehensive guide to the Underdark's exploration routes, crucial quests, and the most vital objective: the Adamantine Forge.

Underdark Major Quests Overview

Accessing the Underdark and Preparing for Exploration

There are multiple main routes into the Underdark, but the most common is through the back of the "Defiled Temple" in the Goblin Camp. Solving a puzzle opens a hidden door, leading you to the Underdark via a Selûne worshipper's outpost. Other methods include descending from the massive hole in the Whispering Depths using a "Feather Fall" spell or Spell Scroll.

The recommended level for exploring this area is 4-5. It's safest to gain a class level to 4 before attempting it. Since there are many poisons and various terrain effects, having "Poison Resistance" abilities / spells or elixirs, and movement abilities / spells like "Misty Step" or "Jump" will significantly ease your exploration.

Key Areas and Unmissable Unique Gear

Selûne Outpost and the Legendary Sword "Phalar Aluve"

Upon entering the Underdark from the Defiled Temple, you first arrive at the ruins of a Selûne worshipper's outpost. To the west of this area (near coordinates X:116 Y:-191), you'll find "Phalar Aluve," a beautiful longsword impaled in a rock. This is an incredibly powerful unique weapon and an item you definitely want to acquire.

1. Approach the sword impaled in the rock and interact with it.
2. A Strength or Religion ability check will occur.
3. If the check is successful, you can pull out the sword. Failing will deal damage, but you can try again.

"Phalar Aluve" possesses a special action that can either buff allies or debuff enemies within an area centered on yourself. It is particularly well-suited for classes that combine front-line combat with support, such as Bards and Paladins.

• Hidden Element: A gigantic eyeball monster, the "Spectator," lurks nearby. Freeing the petrified Drow it guards will yield important information regarding the Adamantine Forge.

The Myconid Colony: An Exploration Hub

Near the center of the Underdark lies the "Myconid Colony," home to friendly mushroom folk. This serves as a safe resting place, complete with merchants and quest givers, making it an ideal hub for exploration. You can accept major quests here, such as "Defeat the Duergar Intruders" from Sovereign Spaw and "Find the Missing Mushroom Picker" from the merchant Derryth.

How to Craft the Weapon "Mourning Frost"

In the Underdark, there exists a very powerful staff, "Mourning Frost," which is completed by gathering and crafting three parts. This staff enhances ice damage and can be a contender for a final weapon for Wizards and Sorcerers who primarily use cold-based abilities / spells.

1. Icy Handle: Obtain it as a reward from Sovereign Spaw of the Myconid Colony for completing the quest "Defeat the Duergar Intruders," or find it on a Drow corpse.
2. Icy Crystal: Dropped by an enemy named Filro the Forgotten in the Sussur Tree area (near coordinates X:-51, Y:-149).
3. Icy Metal: Found on a Drow corpse deep within the poisoned area, which is the destination for the quest "Find the Missing Mushroom Picker" accepted in the Myconid Colony.

Once all three parts are gathered, right-click any one of them and select "Combine" to complete "Mourning Frost."

The Adamantine Forge: Critical Exploration Area

The ultimate goal of your Underdark exploration could be said to be the "Adamantine Forge," located in the deepest part of Grymforge. Here, you can craft "Adamantine gear," which boasts some of the best performance in the entire game.

Adamantine Gear Crafting Steps

1. Find the Moulds: Six types of "moulds" are scattered throughout the Grymforge area: Scale Mail, Splint Armour, Shield, Longsword, Scimitar, and Mace. Be sure not to miss the heavy armor moulds in particular.
2. Mine Mithral Ore: Mine two "Mithral Ores" from the glowing bluish veins found in the Underdark. One is located in the southern part of the Underdark (near the Arcane Tower), and another in the eastern part (near the Ancient Temple).
3. Activate the Forge: Place a mould into the central "crucible" of the forge and a Mithral Ore into the slot next to the "Mould Release Lever."
4. Flow the Lava: Pull the forge's lever to lower the platform, then operate the nearby "Lava Valve" to release lava. This will cause the boss, "Grym," to appear.
5. Defeat the Boss "Grym": After defeating Grym, operate the "Lava Valve" again to release lava, and finally pull the central "Forge Lever" to bring down the hammer, completing your gear.

Tips for Defeating Boss "Grym"

Grym is an incredibly tough boss, but he has a clear weakness: he only takes damage while standing in lava. Furthermore, you can defeat him relatively safely by utilizing the forge's mechanisms.

• Grym has a habit of targeting the last character to attack him. Lure him to the central crucible with a character capable of ranged attacks.
• Once Grym is on top of the crucible, operate the "Forge Lever." A massive hammer will drop, dealing immense damage to Grym. Repeating this twice is the most efficient tactic.
• Grym is vulnerable to Thunder damage. If dealing damage with abilities / spells or arrows, utilize those with Thunder properties.

Secrets of the Hidden Area: "Festering Cove"

The Underdark contains a hard-to-find hidden area called "Festering Cove." This place is a treasure trove of unique items and equipment, so be sure to visit.

Access Method

On the cliff edge south of the Arcane Tower (near coordinates X:-45 Y:-211), there's a "cracked wall" that can be destroyed to gain entry. However, entering requires jumping across a gap, so abilities / spells like "Misty Step" or "Jump" are useful. Failing the jump carries the risk of a party wipe from fall damage, so quicksave beforehand.

Acquirable Unique Equipment

This area is inhabited by a fish-like race called the "Kuo-Toa," whose events progress based on their object of worship. Depending on your persuasion, you might acquire the powerful sickle "BOOOAL's Scythe." Additionally, skeletons and chests within the cove may contain useful armours such as the "Helmet of Autonomy," which grants advantage on Saving Throws, and the "Slippery Chain Shirt," which makes enemies more prone to falling prone.

Summary

The Underdark is a vast and dangerous area, but it offers an adventure well worth the risks. The gear crafted at the "Adamantine Forge," in particular, will be a significant aid in overcoming the tough battles of Act 2 and beyond. The priority items and equipment introduced in this article are as follows:

• Adamantine Splint Armour: Critical hit nullification offers top-tier defensive performance.
• Phalar Aluve: A powerful longsword with offensive and defensive buffs/debuffs.
• Mourning Frost: A staff that significantly boosts the damage of cold-based casters.

Gather this equipment, traverse the Underdark fully prepared, and proceed to your next adventure.