This mob spawns at 58851, 25035 in Norsca at the back of the Pit of the Forsaken PQ. Look for three bright lights in the sky.
You do not have to complete the PQ to kill this mob. If it is up, you will see a magus called Thuldu Ebonweal in the middle of six Forsaken Acolytes.
What follows next is a series of stages that are almost never the same, except in the quantity and quality of opponents.
Next 'stage' is the same amount of Raging Furies - or similiar. Then 3 Raging Bloodletters - or similiar. After that a Hero like a Searing Changebeast spawns. Then, Goetius the Withered spawns or similiar (Vokrakx the Conqueror might spawn, for example). He has a lot of Hit Points, and is a level 9 lord like the one at the end of the Pit of the Forsaken PQ, so be prepared with potions if you think you need them.
One note: These mobs are timed. They do not spawn because you killed the previous mob. Take too much time and you will be overrun.