This is the Relic unit for Chaos. It's a greater daemon who's a dedicated (giant) melee fighter with jump ability using his wings. The Bloodthirster is built from the Daemon Pit, but doesn't actually appear there. Instead, once he's "built", he's activated from a commander or squad leader who is then killed as a blood sacrifice, and the Bloodthirster will appear there. Hence one of these could show up unexpectedly in the middle of a fight. Killing one isn't much different from killing an Avatar; you dance with, say, an Assault Squad while focused fire wears it down. Your commanders can hurt it pretty good as well.
He constantly loses HP (at a very slow rate) when he's not fighting, but regenerates HP when he's engaged in melee combat.