Hi everyone! I'm updating the solution after realising how plain and dumd it was.
To get to this part you need to complete four different prophecies.
The unbreathing queen
The plaguemaw
Unbearable whisper
The feral lord.
Note that all four of these have five different stages and when you complete the fifth one you get a map piece.
Prophecies are random and I don't really know if you can manipulate this in any way. But you can check the trading board for both prophecies and map pieces.
The map pieces you are looking for are,
Volkuurs key
Ebers key
Inyas key
Yriels key
When you enter this map you end up in a small room leading into the fight. There are four different bosses in this are and you can only fight one of them at a time. After a while the first one will shield himself and the next one will start to attack you, at the same time there will be mobs of enemies spawning as well. When i unlocked this achievement i only killed one of the bosses and i don't know if this is common or not.
I will include a link to the wiki for PoE and I hope this clears up any confusion.
https://pathofexile.gamepedia.com/The_Pale_CourtCheers!