Ugh jar of blood

[Som1Lse]

There are two posibilities.

The first is in the transcept in, I think, the right torture room, but Im not shure.
There is a grate and below it is a lot of blood.
Combine the glass jar with the string (if you havent already) and use it on the blood. (you will have to open the grate)

The transcept isnt availiable after you get caught, so the developers also made a second solution.
In the nave after you are caught a new room is open.
First navigate to agrippa and then look to your left.
There is a hole in the wall and behind it is a room.
There is a grate and below it is a lot of blood.
Combine the glass jar with the string (if you havent already) and use it on the blood. (you will have to open the grate)

(C&P FTW)