Arathremia  

Arathremia has an ability called 'sleep', which she will use randomly on members of the party. This takes you out of the fight for 5 seconds. She also periodically calls Deceived Souls to help her. These are non-champions and can easily be off tanked. She calls them quite often however, much faster than the cooldown on challenge, so you have to be careful since your tank might have a hard time catching them all. When she gets to low health, the minions turn on her and help you kill her... it's actually rather funny to watch.

Arathremia has a rather short leash - if you go down the ramp, she will reset, despawn and respawn after 1 minute. You have to fight where she stands.