The Cantigaster is a beast locked away in the Shuttered Palace's cellars, which possesses the deadliest venom known to man. Cantigaster venom can bring a more permanent form of death than that usually suffered by residents of the Neath, and it works best when applied to a blade.

Correspondence1 Starting a department at the University

The Senior Reader in ______ has been poisoned by Cantigaster venom. The subsequent investigation reveals the truth about the Cantigaster: he is the husband of the Duchess, bitten by an asp before the fall of the Second City in Egypt. The Duchess bargained with the Bazaar to save her beloved's life in exchange for her city; he lived on, but in a tormented state, his flesh eternally bloated with poisons. The Duchess still visits him at midnight each Sunday in the third wine cellar under the Shuttered Palace to milk venom out of his skin and bring him relief; what she takes away, presumably, is Cantigaster venom.

This was common speculation for a while based on the events at the University, but it was only confirmed in "The Calendar Code" that the Cantigaster is in fact Tutankhamun.

Cheeryman The Cheery Man...
...was poisoned with half a drop of Cantigaster venom, which left him barely able to walk.