The Death Demon monster normally attacks and shows a "floating combat text" above the head of its target when it hits. Like all units do. However, if it is the special animation used when the Death Demon performs a killing blow, no numbers are shown at all.

This is particularly confusing because the Death Demon also causes cleave damage (damage on all adjacent targets to true target). This text ALSO does not appear (when the demon kills its true target).

Furthermore, any target that dies from the cleave damage is not killed instantly. Instead, it remains at 0 health and gets to act one more turn. Then, it falls down dead at end of its turn.