In the cathedral you come across crypt of Lucian. Here you have to solve a pipe puzzle to open a secret hatch.

You need to do following steps to solve the puzzle
use blood rain on the iron grate so that blood is transferred to the other side
Interact with altar and select second option.This allows you to play with the pipe circuit. complete the circuit such that blessed Fire/ Water and Blood at source end could flow to the destination.
Once the circuit is complete, bless the blood on the iron grate and immediately interact with altar and select option 1.

Now the blessed Fire, water and blood will flow to their respective source and open a secret hatch.

Arx Quests Playlist:    • Divinity Original SIn 2  Arx Quests