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Travel through the Underdark
Travel through the Underdark is a sub-quest of Travel to Moonrise Towers in Act One of Baldur's Gate 3. It can be acquired after speaking to either Halsin or Minthara after resolving 
by siding with either the tieflings or the goblins.
Objectives
Objectives and journal entries may vary pending story decisions and outcomes.
- Halsin told us that there is a way to Moonrise Towers that leads through the Underdark. Tunnels beneath the temple of Selûne will lead us there.
- We learned that the best way to get to Moonrise Towers is through the Underdark. Tunnels beneath the temple of Selûne will lead us there.
- We found the old temple of Selûne. The entrance to the Underdark must be here somewhere.
- A room in the old temple of Selûne contains a secret entrance to the Underdark. Through it, we can reach Moonrise Towers.
- We unlocked a secret door in the old temple of Selûne. It leads to the Underdark.
- The Underdark is a twisted labyrinth of tunnels and ruins. To find the path to Moonrise Towers, we'll need to explore.
- We learned that the Absolute sent agents to Grymforge. There must be a route connecting Grymforge and Moonrise Towers.
- We discovered a lift in Grymforge that can bring us closer to Moonrise Towers. However, we were warned of a darkness that plagues the surface.
Reach Moonrise Towers through Grymforge.
- We used the lift in Grymforge and arrived in the shadow-cursed lands.
- We decided to take the mountain pass. This road should lead us to Moonrise Towers.
Walkthrough
NOTE: The party does NOT need to choose between exploring the Underdark and the Mountain Pass. Both can be visited before going to the Shadow-Cursed Lands.
If the party meets with Halsin as part of , he explains that he cannot heal the party, but that he's been studying the Absolute cult's activities and knows they are sending people en-masse to Moonrise Towers. He suggests that the party head there to seek answers. During Save the Refugees, if the Druid Grove was protected, Halsin speaks with the party after a night of celebration. He explains that there are two paths to Moonrise Towers - one through the Underdark and one overland through the Mountain Pass. The area around Moonrise Towers is known as the Shadow-Cursed Lands, and has a dangerous curse that can drive people mad. Halsin recommends the Underdark route, as it means traveling over less of the areas infected by the shadow curse.
If the party allies with Minthara, she tells them only about the Mountain Pass, providing a 
Spider's Lyre so that the party can summon one of her minions to cross the Shadow-Cursed Lands overland. She states that the party must go to Moonrise Towers to find their pace in the Absolute's Grand Design. However, the party is still free to go to the Underdark instead.
Finding an Entrance
There are several ways to enter the Underdark. Only one of these paths needs to be pursued to enter the Underdark.
Defiled Temple
One path to the Underdark is through the Shattered Sanctum in the Goblin Camp. The entrance can be accessed through Priestess Gut's chambers, but is guarded by her ogre servant Polma. If talking to Gut about a cure for the tadpole infection and then following her to her chambers, she says that a special cleansing potion must be taken first. However, an Arcana check reveals this is a 
Potion of Sleep. If she's called out on this, or if the party member in question is resistant to sleeping potions (ie a Drow, Elf or Half-Elf), she becomes hostile and also triggers several nearby goblins to become hostile.
However, if the potion is taken (and works), she chains up the party member and says she intends for them to become her pet squid.
It's possible to break free with either an Acrobatics or a Strength check, but if nothing is done at all, Korrilla appears from a portal to assassinate both Gut and her Ogre guard. Then she disappears after a brief conversation, warning the party to be more careful.
Once in the Defiled Temple, there is a puzzle blocking the way forward. There are four overlapping circles. The goal is to get the black circles into the southernmost circle. Follow the combination to unlock the door:
- South
- East
- South
- North
- East
Alternatively it is possible to find a lever on the right side of the door near the stairs upon a successful perception check. It can be then unlocked using lockpicks.
Go through the now-unlocked door and climb down the ladder to enter a Selûnite Outpost. The gate forward is closed and guarded by moon magic, but can be opened by using ranged attacks on the Moonstone held by the Selûne Statue. Another option is to exit out of the broken window on the western wall of the outpost.
Whispering Depths
There are two ways to enter the Whispering Depths. One way is to enter the blacksmith's basement north of the Blighted Village Waypoint (this is also the same basement where is completed). One of the northern walls is cracked and can be destroyed with force damage, which leads to the Whispering Depths. The other way is to investigate the well (X:23, Y:400) north of the Waypoint. Choose "[INVESTIGATION] Peer into the well" and then climb the bucket rope ladder down into the well to arrive at the cave.
The chasm to the Underdark is by the Spider Matriarch's nest (X:-558, Y:-360). In order to jump safely from the chasm, the party must use 
or a similar spell. Otherwise, the party's bodies are shattered upon impact from the massive height.
Zhentarim Basement
To use the Zhentarim's entrance, their hideout must first be located. It's in a barn west of Waukeen's Rest, at the shabby door (X:-113, Y:613). The party will automatically be aware of the Zhentarim Hideout depending on if Rugan and Olly were encountered as part of - either by speaking with Rugan or by asking his corpse questions with 
. If the party spares Rugan's life, he shares the location of the hideout along with the password need to enter. With a successful perception check, the party notices a man, Salazon, crouching behind some shelves. Reciting the password 'Little serpent, long shadow' has him stand down immediately and provide the key to a wardrobe that leads into the hideout. If the party does not have the password, there are a myriad of other options to get past Salazon:
- Read his mind.
- [PERSUASION] Just calm down - I'm not here to fight. (DC 15)
- [ROGUE][PERSUASION] This is all pretty crude, as traps go - you'll probably kill us both. (DC 15)
- [SORCERER][PERSUASION] I am magic. Cast that spell - you'll only hurt yourself. (DC 15)
- [BARBARIAN][INTIMIDATION] Careful. You won't kill me, but you will make me mad. (DC 15)
Succeeding to make the man stand down allows the Zhentarim to be dealt with peacefully. Follow his instructions to go down the hatch and through the wardrobe to enter the Zhentarim Basement. Once inside, follow the path forward and climb up the ladders to where Zarys is. Move north and cross the bridge. A man named Karad has a mechanical key and guards the way further. At this point, if the party goes onto the bridge, the Zhentarim become hostile if they are spotted. Either use stealth skills to pickpocket Karad's key and move past him, or kill the Zhentarim and loot the key. Slightly north of Karad is an illusory wall (X:268, Y:-189) that can be detected with a Perception check. Move through the wall and use the key on the Elevator Winch, then use the elevator platform.
The Hag's Basement
The party must enter Auntie Ethel's teahouse in the Sunlit Wetlands (X:-47, Y:266) and use Ethel's illusory fireplace to enter into the Overgrown Tunnel. The first hurdle is a door of vines. Attempting to use it causes it to telepathically share its story with the party to warn them from proceeding further. An Arcana check reveals that the door is an illusion and can simply be walked through. Otherwise, persuading the door (DC 10) causes it to share its secrets.
Through the door are four servants of Ethel's - Mask of Terror, Mask of Vengeance, Mask of Regret, and Mask of Servitude. The become hostile upon detecting the party. On the table next to the door are 
Whispering Masks. If worn it prevents the Hag Servants from becoming hostile. However. while Auntie Ethel is alive, the wearer of this mask must make a 
Wisdom saving throw every turn. If failed, they become a Hag's Pawn (controlled by Auntie Ethel) and will attack the party; if all party members equip the mask, the Game Over screen appears. If all of the unaffected party members run away from combat (e.g. back to the Tea House), leaving no one to attack, the remaining mask-affected party member will be able to unequip the mask after making a wisdom saving throw.
Past the Hag Servants is a waterfall and a winding path filled with noxious fumes and explosive traps. On this path, there is a door to the north (X:-354, Y:577) that can only be passed while wearing a Whispering Mask. Pass through the door and move to the Mushroom Circle to enter the Underdark.
Getting to Grymforge
Once the party arrives in the Underdark, they must make their way forward to try to find a way up into the Shadow-Cursed Lands. There are many helpful quests in the Underdark, including a main quest to attempt to remove the tadpole - - but only two quests in particular directly assist in moving further into the Underdark. See the Underdark Quests for more information on optional quests the party can complete.
The first of these quests is , which allows the party to kill the duergar at the Decrepit Village (X:11, Y:-195) and steal their boat to get into Grymforge. The second quest is , which involves working with the duergar to track down a missing gnome who stole some boots, and then being charged to deliver the boots to Grymforge. The party can also attempt to sneak past the duergar and sneakily access the boat without doing either quest.
Once in Grymforge, the party is tasked with many different objectives, including freeing a True Soul drow named Nere as part of . See the Grymforge Quests for information on optional quests. However, the party can almost immediately ascend to the Shaadow-Cursed Lands upon entering Grymforge. East of the Grymforge Waypoint is an elevator (X:-603, Y:382). Interact with the Wrought-Iron Gate to the elevator and the party is stopped by a duergar named Ward Magmar. Magmar asks the party if they have one of Nere's 
Moonlanterns and explains that they risk succumbing to the shadow curse without one. Tell him the party isn't concerned about the curse and proceed forward to use the elevator. If the duergar are gone or dead, then a gnome named Skickpit initiates the same conversation.
Consequences
Entering the Shadow-Cursed Lands is considered the "point of no return" for Act One and is the official start of Act Two. As such, using the elevator in Grymforge means that several quests automatically update. Before entering the elevator, complete all desired quests.
Notes
- Both the Mountain Pass and the Underdark have unique quests and loot. For the most complete playthrough, it is recommended that the party explores the Mountain Pass, Underdark, and Grymforge before going to the Shadow-Cursed Lands.