shiny brass halber named in citadelFollow

He's a triggered spawn. First you must kill the group that rare spawns in the center of the citadel. Check all the instances if you can. This is the hardest part of the quest, and you might have to wait awhile for him to spawn.

Normally the courtyard inside is more or less empty. If a group spawns there, destroy them, and as soon as the last falls T'Snaee and his crew pops and instantly engages you, so be ready.