I'm in the soul cairn and i need to retrieve the soul essence gem. I'm directly under it in some maze of a building and i don't know how to reach it.

theres a small 'well' , you jump in it and it teleports you to the roof where the chest with your soul essence will be, its a bit of a pain to find but it has one of those giant soul harvesting crystals above it so grab your soul back and run haha

Eating some of the soul husks before going to the chests helps with the health drain