Terrorcon

Born from dead Transformers infused with Dark Energon, the Terrorcons are Megatron's greatest asset—an undead army under his thrall. Each is powerful and relentless in its own right; en masse, their persistence and Dark Energon they radiate can sap the strength of their Autobot prey until they can pull it down and tear it apart. Thankfully, they are not invulnerable. Energy blasts of sufficient force can deter them, though the best method is to use bladed weapons to slice them to pieces—the smaller the better—as the Dark Energon charge cannot sustain itself within the smaller, disparate components. Simply slicing them in half doesn't seem to be sufficient, though the method of irradiation may be a factor: The one case known to be prodigiously active despite total bisection was the product of a direct implantation of crystallized Dark Energon; all others to date have been created by absorbing energies emitted over a wide area by a shattered crystal.

Ficiton
After Cliffjumper was killed after a battle against the Vehicons, his body was saved by Starscream for use in experiments. When Megatron returned, he wished to test the then-theoretical property of Dark Energon to revive the dead. It worked, to an extent. Cliffjumper was reborn as a screaming beast, and he promptly destroyed two Vehicons before leaping at Megatron. Megatron commented that the creature before him was no longer an Autobot and sliced the Terrorcon Cliffjumper in half with his claw. The half with the head remained alive, and it attacked Arcee when she tried to rescue it.

Megatron realized the potential for brutal, undead warriors from the encounter with Cliffjumper and decided to use the Dark Energon more extensively. When Optimus Prime realized that the substance they were dealing with was Dark Energon, he and Ratchet put two and two together and realized Megatron was up to something, that something being raising the dead. Eons ago, there had been a war on Earth after the smuggling of Energon to the planet. Since Cybertronian bodies do not decompose, there were some still buried in the ground. Megatron appeared to Optimus Prime and Ratchet in a large canyon and threw a chunk of Dark Energon into the ground, which promptly began to glow bright purple, as did Megatron himself. The ground opened up, and undead robots crawled out from the cracks, brandishing the same deadly weapons they'd carried in the previous war.

Despite being outnumbered, Optimus and Ratchet were able to dispose of the Terrorcon horde, only to be informed by Megatron that this was merely the prelude to his main event. That event, they soon discovered, was raising the countless dead on Cybertron itself.

Unfortunately for the Autobots, Megatron was already a step ahead of them in implementing his plan, since he already had a working space bridge and a ship to get to it. The Autobots used their GroundBridge to reach the space bridge, but failed to stop Megatron from throwing his supply of Dark Energon through, raising the entire deceased population of Cybertron. Before the Terrorcons could pass through the bridge, the Autobots managed to reverse the power flow, causing the bridge to explode, taking the zombies and Megatron with it.

Starscream later used a shard to attempt to revive the slain Skyquake. While he successfully raised the mighty Decepticon, due to a series of unexpected events the revenant Skyquake was left trapped in an alternate dimension.

The addition of Dark Energon to Knock Out's Synthetic Energon formula turned Cylas into a new form of Terrorcon which craved energon. His bite passed on the infection, causing a brief plague that was swiftly quelled by the Decepticons. Airachnid, however, was sent to one of Cybertron's moons via space bridge after having been infected by Cylas, and presumably spread it to the Insecticons she fed upon.