The Mercykiller wyrm is a breed of wyvern bred over the last centuries, kept first by the Sodkillers and then, following the Great Upheaval, the Mercykillers. Bred for ferocity, these creatures are kept on the Outlands at a secluded location, with a single wyrm — traditionally named by the faction "the Great Green Wyrm" — kept in Sigil at the Tower of the Wyrm for prisoner intimidation and, at times, execution. All Mercykiller wyrms are carefully watched over by the faction, with any attempts at harm, theft, or harassment quickly responded to with full force.
These creatures are the inspiration for the symbol of the Mercykillers; massive serpentine beasts almost 30 feet long, green-scaled, with thick hide and nearly vestigal wings. Even the creatures's venom has been altered; over many generations of breeding, its venom now works as a truth serum, rendering those suffering from its effects unable to avoid the temptation to answer any question posed to them with the truth. As of late, even its blood has been found to hold the same properties when specially alchemically treated into a substance known as blood of justice.
While the Mercykillers do take much advantage of this substance, they are far more known to the people of Sigil as one of the three forms of execution given to those sentenced to death. Such execution is a rare occasion, given only to traitors to the city, and when ordered this event is without question declared a public holiday by the Mercykillers, that all can come and see justice done as the Wyrm savages the bound and blindfolded felon in the Prison square.
- Factol's Manifesto, pg.109
- In the Cage, pgs.21,44-45