With those crystals, first you get some normal sized brittle skeletons, then double that number Huge and then double again Reavers. Then the Crystal can't be used again.
There's so many Reavers that you'd probably need the Kegs or Exorcism to kill them all before they disappear, but they're pretty easy at high levels.
I was just surprised to see high-level treasure on them.
My Exorcism skill is still very weak, I think it only does 10-15 damage, anyway I'm into the Crossroads army now, not having too many problems, I think I can kill off 3 at a time without resorting to healing potions or a henchman.
