Pit Commander

The Pit Commander is an annihilan. It can be seen commanding the Burning Legion force at the Outland side of the Dark Portal.

Background
He is the demon in charge to enter Azeroth, he commands the "Marauding Agents of Evil" that consists in Infernal Siegebreakers, Fel Soldiers and Wrath Masters.

News From Outland: Crisis at Da Portal!
The Pit Commander makes a brief cameo where he is seen sending waves of Fel Soldier.

Strategy
Remember that the Pit Commander is there "for show", like everything else on the stair, and was not intended to give players any reward for killing him. However, he can be attacked and killed, though he leaves no corpse and is therefore not lootable, nor is any quest or achievement satisfied by his death. His abilities include summon Infernal Siegebreakers and an abnormally (and likely intentionally) powerful Rain of Fire, and stepping on players will cause them to take damage, as well. To be clear, his Rain of Fire will hit a level 80 plate class wearing tanking gear for about 60000 damage.

Pulling the Pit Commander onto the Stair will cause the Alliance and Horde soldiers to attack him, but only if they are not in combat with other demons. It is a solid strategy to aggro him from as far as possible, then run to the Stair, while no other mobs are present, and the Alliance and Horde soldiers will take him down in a matter of seconds. Note that he will not lose aggro on you unless he steps into an Alliance/Horde NPC's AoE attack, so if leading him to one is not possible, run to the Dark Portal or dump aggro in whatever way is possible.

When killed, the Pit Commander will respawn near Hellfire Citadel, apparently instantly, then walk up the Path of Glory and return to the Stair.