Bloodrain is a noxious, magical substance that is poisonous to any human-being or animal that touches it or ingests it. It takes a long time to brew properly, and even then it is dangerous to the potioneer who creates it, as they could accidentally touch it.
Bloodrain was first heard of in Wolf-Speaker, when it became known that Gissa of Rachne was brewing it in order to use on the Tortallan forces that had arrived outside the barrier in order to stop the treasonous plot from unfolding. During the brewing process, Gissa accidentally got it on her hand, and it was so painful that she had to cut her own hand off.