Larian Studios Forums Divinity: Original Sin 2 Divinity - Original Sin 2 - General Lucian's Crypt puzzle is funny

The first dialog option start the process the second let's you adjust the flow.
After adjusting the flow (2 option) and before you choose the first dialog option you need to cast blood rain or similar and bless it.

Be careful not to undo the holy fire I re comment Quick saving before as sometimes i can take a few tries.