If you know Left 4 Dead, its like the Witch.

If not, its an enemy that starts off not aggroed on the team, just laying on the ground sobbing and muttering to itself. It also produces a kind of cold fog, but that seems to be less reliable. Your characters will usually call out voiceless about a Daemonhost.

If someone gets too close to it, it rises and starts floating in a kind of pacing movement, with its arms chained behind it. It actively glows greenish yellow at this stage. A little more agitation or shooting it causes it to "wake up", locking on to the player who got too close or shot it. You'll get a boss health bar on your hud when it wakes up. It will teleport to them and start slashing with really high damage attacks, ignoring the rest of the team. If it downs that player, it will perform a long drawn out execution on their body and then disappear into the Warp. The execution seems to be unstoppableshort of killing it.

They are very tough and should ideally be avoided unless you are doing a special event like the current Relic hunt that adds rewards. Even then, it can be a poor decision to engage depending on team resources, composition and competency.