After you have progressed in Act I, you will eventually need to make your way to the Underdark and there are multiple ways to get there.
One of these is by heading to the Selunite Outpost, which was built a long time ago as a secret stronghold where the Dark Justiciars (Shar worshippers) were rallied.
Making your way to the Selunite Outpost is one of the easiest ways into the Underdark and there is a decent amount of loot for you to grab as well.
Where is the Selunite Outpost?
The Selunite Outpost is around the southeast part of the Underdark, which can be accessed through the Defiled Temple within the Shattered Sanctum.
The Shattered Sanctum can be reached by heading to the Goblin Camp and this is where you will initially find the Goblin Leaders.
How To Reach The Selunite Outpost?
After you have entered the Shattered Sanctum, make your way to Priestess Gut so she can lead you to her room where you can kill her or allow her to capture you.
If you are captured, you will eventually be able to escape and will be able to roam the Defiled Temple or you can get in by lockpicking the door in her room.
Continue making your way to the puzzle room, where you can open a path by solving the puzzle or by using the lever nearby. (Require Perception Check)
This will cause a large doorway to open, which leads you to a ladder that you can interact with that will bring you to the Selunite Outpost.
Notable Loot
Once you are in the Selunite Outpost, you can start looting the area and will be able to find multiple items, including the following:
1) Helmet of Smiting
Found inside of a Gilded Chest, this helm grants the wearer Temporary Hit Points when a condition is applied to an enemy while casting a Smite Spell on them. (163, -244)
The Gilded Chest may be found beyond the Iron Gate on the right when you descend from the ladder. (The key to the gate can be found on Skeleton above the main gate)
2) Luminous Armor
Obtained from Opulent Chest, this armor causes a Radiant Shockwave to be released when you deal Radiant Damage (176, -247)
You may find the Opulent Chest in a hidden room, which can be accessed by passing a Perception Check and interacting with the Stone Wall near the Gilded Chest.
3) Staff of the Moonmaiden
Found outside of the Selunite Outpost beside one of the Skeletons, this Staff does not have any special properties.
Verdict
The Selunite Outpost is a great way to start your journey into the Underdark and it is one of the top choices when it comes to reaching the area.
Grabbing the loot in the Selunite Temple can provide you with useful items to use or sell if you need extra Gold later.
While the Selunite Temple is secure, you can find your way into most of the rooms without trouble since you can lockpick most of the doors and chests.