The gronn Slaag is one of the seven sons of Gruul the Dragonkiller. He exiled himself from the Blade's Edge Mountains and took control of the Gordunni ogres that reside in the Barrier Hills just outside of Shattrath City. He is one of the three Gronn who Mog'dorg the Wizened asks the player to kill.
Slaag has an occasional AoE knockback which 'pulses' for about five seconds, hitting continuously for high damage during that time (this is similar to the one used by Princess Theradras in Maraudon, except that he can move while using it and will follow the tank.) A well-geared tank should not be in much danger, but melee DPS may want to move away when he starts the attack. Healers and ranged DPS should stand far enough away to avoid the attack entirely. The building he is in is large enough to avoid the attack with no difficulty.
Patch 2.1.0 (22-May-2007): Added.