Cortex Helm
From Transformers Wiki
An artifact of Onyx Prime, the Cortex Helm may look like an ordinary crown, but it possesses the power to reprogram Cybertronians to the likings of its wearer. And if its wearer's liking is mindless drones, it can enable control over them with mere thought. To keep it from falling into the wrong hands, it was sealed away eons ago, its location known only to Onyx and his servant, Thunderhowl.
Fiction
Cyberverse cartoon
To prevent its power from being misused, Onyx Prime elected to seal the Cortex Helm away. He built a flying, invisible temple and guarded it with sentry drones disguised as statues of the Primes. Within the temple was a sealed door that could only be opened with the Matrix of Leadership. Onyx entrusted his servant Thunderhowl with the location of the temple and the Helm, and had him swear to keep it from falling into the wrong hands. The Cortex Helm was hidden away so well, even Optimus Prime was unaware of its existence.
After the end of the war, an army of drones from another universe showed up on Cybertron and began fighting one another. One of their number who had gained independence, Tarn, located Optimus and told him that the Cortex Helm was the only thing that could bring the Perfect Decepticons under control. Optimus agreed to help Tarn, and took him to see Thunderhowl. Thunderhowl explained that the Helm had been hidden away, and was hesitant to take them to it, until Optimus assured him that it would be used wisely. Thunderhowl took Optimus, Tarn, Bumblebee, Soundwave, and Shadow Striker in a ship to the invisible temple. Upon arrival, Thunderhowl led the group through the security measures to the room where the Helm lay, but as soon as Optimus used the Matrix to open the door, Tarn rushed in and put the relic on his head. Shockingly, Tarn had been lying about his intentions; instead of making the drones stop fighting, he reprogrammed them to serve him.
Tarn immediately used his new army to capture Optimus Prime and bring him to the Cube arena. There, Tarn set up a few of his soldiers for Optimus to battle, with the intent of having the Autobot leader fight his way up to facing him. Simultaneously, he had the rest of his drones attacking Autobots around the planet, while imploring the native Decepticons to join in.
Tarn kept his little cage match going for some time, until a team of Autobots teleported in to try and save their leader. This led to Tarn ordering Shadow Striker and the native Decepticons to attack, only for Shadow Striker to tell him to get off of Cybertron. Tarn then used the Helm to order his drones to attack the Autobots and Decepticons. Hot Rod attempted to remove the Cortex Helm from Tarn's head, but was unsuccessful.
With the battle in the Cube arena going poorly, Tarn summoned more of his drones from around the planet to overwhelm his foes, but it wasn't enough. Hot Rod distracted Tarn by making another grab for the relic, but it was ultimately Soundwave and Laserbeak who managed to get the Cortex Helm off of Tarn. Donning it himself, Soundwave reprogrammed the Perfect Decepticons to give them free will, before Tarn punched him in the face and took the Helm back. But before he could put it back on his head, Soundwave unleashed a sonic blast so powerful that it utterly obliterated Tarn, the Cortex Helm, and himself. The Perfect Decepticon
