His initial location is near the main entrance to the instance. Talk to him, and he skips away. When he stops, talk to him again. After repeating this process three more times he becomes hostile and summons 4 level 56-57 non-elites, who do fire damage. Note: You'll see Pusillin at this moment growing immense and gain a buff called "Spirit of Runn Tum". Casters trying to solo him for the key, beware, it will reflect most of your spells back at you. It is advised to kill him with either a wand or AoE effects.