Underbog Colossus are single pulls that come between the Type 1 packs early in the instance. Like the Ancient Core Hounds in Molten Core or the Anubisath Sentinels in Temple of Ahn'Qiraj these mobs pick from a random set of abilities. They will retain whichever set they have until respawn, when they will reset. Aside from their random abilities, each one has a elemental weakness. Unlike other such vulnerabilities, these ones cause any spell of that element to always critical strike if possible.