The Scaven is a type of undead creature created by the Night Watchman and commanded by him (until they turn against him). Scavenger's by trade, they scour the castle in search of those who have died within the walls and eat portions of their flesh, the rest of the body is mutated into more Scavens, and with no shortage of corpses within the castle walls, these beasts have the potential to expand without limit.
There is a mutated version of this creature, called Flying Scaven. This is basically a winged version of the common Scaven that also appear in Mirror of Fate, said to be the result of a mutation of the Scaven.
"These reanimated corpses eat and infect the dead remains of other creatures, with many sections of the castle threatening to become infested with them.(...)"
