The War Drum is an instrument found in the temple of KT. When hit, it will make any Creatures that are near it extremely angry and will cause them to hit or even kill each other, making it a very dangerous toy indeed.
The War Drum's effects do not wear off on their own. The Medicine Rattle released with the Bone Cave acts as an antidote to calm all Creatures within earshot.
A second, makeshift solution is to open up the CAOS Command Line by pressing Ctrl+Shift-C and type in
enum 4 0 0 urge writ targ -1 2 -1 2 next
to cure all Creatures. Typing
urge writ norn -1 2 -1 2
will cure the selected Creature.