A Dangerous Experiment is a written note found in The Brigmore Witches.
I've come across some old writings that hint at a bizarre ritual, and I'm going to use it to prove to you for once and for all that all of this mystical stuff is just superstition. When I get back from my mission in the sewers we'll do this. But there's a trick; you'll need a bride. Hah! Near as I can tell she doesn't need to be willing. She doesn't even need to be awake. By the Outsider's Eyes, I don't even think she needs to be alive. So you go secure a bride if you can, and I'll officiate this mock wedding.
It can be found in an abandoned apartment near the entrance to Drapers Ward.