If you want to make your way to the Defiled Temple, you are probably trying to get to the Underdark because this is one of the passages that lead there.
There are multiple ways to get into the Underdark but going through the Shattered Sanctum through the Defiled Temple can get you there quickly.
You will of course have to make your way into the goblin camp and will have to get rid of certain threats before you can make it though.
Where is the Defiled Temple?
The Defiled Temple may be accessed from within the Shattered Sanctum, which is done by entering a door found in one of the rooms on the left side of the map.
You can reach this door by heading left and taking the second door, which is to the right of the room being guarded by mercenaries.
Speaking to Priestess Gut will eventually end with her leading you to this room and giving you access to it, but you will need to get in one way or another.
How To Enter Defiled Temple?
You can enter the Defiled Temple by picking the lock on the door that lets you enter the room as well as the door leading into the temple. (This requires Thieves’ Tools)
Another way is to speak to Priestess Gut, who leads you into the room, allowing you to pickpocket the Priestess Key or kill her to get it. (This will allow you to enter and unlock the temple door)
You may also gain access to the Defiled Temple by drinking a potion that Priestess Gut gives you, which knocks you out, and ends up with you being captured and saved. (You will be rescued in the temple)
What is in The Defiled Temple?
The Defiled Temple contains a path leading to the Selunite Outpost, which leads you to the Underdark, one of the areas needed for the main story.
The Underdark contains a new area to explore and more challenges to face aside from being an area where certain quests are also done.
This, however, is not the only way to get into the Underdark but one of the easiest that you can take to complete the main story.
Getting into the Goblin Camp is quite easy as all you need to do is pass a couple of skill checks or you can take a stealthier route.
If you are not hostile to the goblins inside the Shattered Sanctum, you can easily reach the room that leads to the Selunite Outpost.
Each entrance to the Underdark has its path and the Shattered Sanctum is considered to be one of the easiest as you start in a safe location.