"That one, you might want to be careful with that. It doesn't like people..."
"A potion? "
Rustcaller Toxin is not a tool of assassination, but instead a weapon of random terror.
A brace of sleep-related poisons.
..The inn was filled with the sounds of leather creaking and seams bursting, as muscles swelled and grew dramatically. Then came shouts of fury and rage as the mob began charging outside, into what would be a very bloody night.
Destroyer of Dynasties, Killer of Kings, Bane of Broods, Curse of the Ruling Elite!
When you sleep, your soul briefly returns to the place where it was created/goes to when it dies, and seeks nourishment. Everybody's soul is created with a little link (or you could say a greater entity calls it back) to that place so that it can return easily, but upon death, the soul returns via that and annihilates the link as if returns.
Also, as an added thingy, you could say the entity supplying the link, or residue of the links themselves, (whichever) is the source of all arcane power.