Hatchet...

[Bareth]

Never happened before but characters are getting killed by taking some type of damage from Hatchet.
Not showing as melee or spell damage, but you get the message:

You have taken 325000 points of damage.
You have been slain by Hatchet The Torturer!

[Sian]

If you move them during fight its normal iirc...

[Demental]

Sian's right, once engaged any movement in the room sets off the AoE.