Glass jar of blood.

[Xanthos]

If all else fails ------\/

Spoiler below!

Find your way back to Agrippa after escaping the cell.
There should be slime covering 70% of the rooms.
Once back to Agrippa you will notice a hole in the wall to the left of him.
On entering there's a little drainage to the left.
Combine string and the jar.
Dip the combined item into the drainage of blood to obtain your goal.