These guys seem like they have a ton of hitpoints, but they really only have high ac. I usually hit from around 130-170 and i was only able to hit them for 30s 40s and the occasional hit in the hundreds. They take a long time to kill and hit hard too. Have fun with them.
I figured they have around 5-6k hitpoints. Nasty HT. These mobs never run when low, which is good and bad. They also seems to be a wandering mob and they do not see invis.
Deathbringers are shadowknights. They harm touch (I've experienced from 501 to 671 damage), and cast shadowknight DOTs on you. They seem to harm touch every battle, but don't seem to cast other spells too often, but I've had them cast engulfing darkness, shroud of hate, and shadow vortex on me.