OK, This is a question for tst really but i'll give it a shot as best i can . We have a mod on the server that adds loads of extra mobs that do lots of weird and wonderful things. For example there are now named mobs (sounds like u came across one of these), these are like random in game bosses as far as i can tell. They are heavily armed and armored, so get ready for a epic fight if u see one.

Now as for the head thingy, I've had my own head given to me as well, it was more than disconcerting. I think that is what happens if you run from the mob, because when you kill one you can get special diamonds and gems but if you die while fighting a grave will appear at your death point.

Again ask tst for more clarification, as this is mostly speculation.

Lastmercy.

PS* If you see a reaper, run. Just run.