Well, that's the oddest title I've written in a while. When attacking Megadeths (which are the special, named Deth monsters), the halo signifying their special status jumps up whenever they do the attacking animation, like this: It's otherwise fine as far as I can tell.
/me goes and checks. For some inexplicable reason the deth attack animations (not just the megadeth, but all the deth ones) are 128x128, instead of 64x64 like their hit, idle, cast, die, and run animations are. Compare! This one is the first frame of deth_atk_d: And this one is the first frame of deth_idle_d: Notice the height difference on the space?
Hah, interesting. This stuff was before my time, but it looks like the animator who made the deth attack animation gave it a larger white-space buffer around the attack sprite, to allow more room for the scythe to swing. Due to how dredmor handles sprites this wasn't an issue positioning-wise, but I guess halos are oriented off the top of the sprite rather than the bottom, which results in this issue. I took a look through the other classic monster sprites and it looks like this is the only one with this particular discrepancy - although some of the hero sprites have this trick being used, so that mod that uses hero sprites for monsters may end up having the same problem. I'll ask dave or nick what they think.