The Harbinger of Absolution  

When you bring this beastie down to a certain percentage of health, you will receive a message saying that the harbringer is splitting it`s energy. When it does, it will spawn two smaller harbringers which need to be killed. The harbringer itself will become unattackable.

Once you have killed the two or three smaller harbringers, you can attack it again.

It seems to do this three times in total. After the third time, you can kill it. When it is dead, Queen Lenya Thex will give a little speech and then float down into the circle, after which she becomes attackable.