A multitude of Orcs have laid siege to the fortifications at Hellfire. Although the attacking warriors are not well equipped, their sheer numbers may spell the downfall of our forces there.
You must take command of the Alliance armies at Hellfire and break the siege before our troops are pushed back through the Dark Portal. We must withstand their charge long enough for the Book of Medivh to be recovered and the rift forever sealed.
The Bonechewer clan's AI is scripted to spam poorly-upgraded basic units, as indicated by the mission briefing. The Shattered Hand and Thunderlord clans have more traditional behaviors, being able to use advanced units like (respectively) death knights and dragons.
While the mission objectives make it sound like you only need to survive until the Orcs give up, they actually never do this, so you must destroy all enemies to win.