Grommash was made warchief of the Iron Horde and Garrosh took his place as Warlord of the Warsong Clan. Eventually, the Iron Horde attempted to invade Azeroth through the Dark Portal. They were able to gain a foothold in Blasted Lands and Upper Blackrock Spire, but were driven back to Draenor by Alliance and Horde forces, who then went to Draenor and fought the Iron Horde.
After a few months of fighting on Draenor, all the warlords (including Garrosh and Zaela) except Grommash Hellscream and Kilrogg Deadeye had been killed. Gul'dan captured Grommash and assumed control of the Iron Horde, corrupting Kilrogg Deadeye with demon blood and appointing him as second-in-command of the Iron Horde. Afterwards, Gul'dan successfully convinced most of the Iron Horde to drink the demon blood and become fel orcs, effectively bringing the Iron Horde to serve the Burning Legion.
Later, Gul'dan flooded Tanaan Jungle with fel energy, corrupting it and turning the Iron Citadel into Hellfire Citadel. He then summoned members of the Burning Legion, including Sargeras' left hand, Archimonde, and began re-constructing the Dark Portal to invade Azeroth.
The Iron Horde was ultimately disbanded when Hellfire Citadel was assaulted by members of the Alliance and Horde. The assault ended with the death of Archimonde, the destruction of the Black Gate, and Gul'dan being sent to Azeroth to start the Burning Legion's third invasion of the planet.