In Transformers Prime, Terrorcons are antagonistic forces that formerly deceased normal transformers (either Autobot or Decepticon) that revived with Dark Energon that manifested from the Chaos Bringer himself.
See: Dark Energon
Despite their savagery, Terrorcons are slow much like typical zombies, making them vulnearable to quick attacks from their foes. The only way to kill these undead beings are via decapitation or severe damage on the head, albeit complete dismemberment are more efficient.
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 blade. 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 after a great deal of time but, 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 dark energon 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 in the Decepticon Vehicons, the only way of destroying them was a shot to the spark. Starscream and Knock Out attempted to handle the plague themselves but Megatron would soon find out. The plague was quelled by the Decepticons but at the cost of over half the Vehicons being destroyed. Airachnid, however, was freed from her stasis lock imprisonment and infected by Cylas before she killed him but she was space bridged to Cybertron's moons via space. Airachnid presumably spread the plague to the Insecticons she fed upon.
Aiming to destroy Primus, Unicron raised an army of Terrorcons from the many remains at a Predacon burial ground. The horde marched to the Well of All Sparks soon facing the Autobots in the Nemesis and the Predacons Predaking, Skylynx and Darksteel. The Terrorcon horde headed into the Well, almost reaching the core before Unicron's anti-spark was trapped and the Terrorcons all disintegrated.
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