Later, however, he lost that arm and obtained his original arm back along with his fusion cannon.
Megatron's fortress of Darkmount was armed with several fusion cannons. When a human army military force arrived at the fortress to wage a counterattack on the Decepticons, Megatron had one of the cannons fired at them, annihilating the humans and destroying the nearby abandoned city of Jasper in the process. Megatron warned Fowler that further human interference would result in the fusion cannons being turned on several major cities.
When Ultra Magnus arrived on Earth and reunited the majority of Team Prime, he and Fowler resolved to disable the fusion cannons before destroying Darkmount with the help of Earth's military. While Bulkhead and Wheeljack created a distraction at the base of the fortress, Arcee and Bumblebee attempted to sneak into the fortress to disable the cannons, but when met with heavy resistance, Magnus took matters into his own hands and mounted a one-bot assault on the cannons. He was stopped by Megatron, and though the Autobots were all captured, Optimus Prime's arrival quickly resulted in the fusion cannons being crippled, and then destroyed by Fowler's Skystrikers.
After being killed, he lost it again upon being resurrected by Unicron, though he was able to access two Dark Energon cannons that were also extremely effective.