News From Outland: Crisis at Da Portal!
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.