Nega-Core

Hey, look, it's made of Kryptonite!

The Nega-Core is an ominous device developed by Jhiaxus, which will enable the coming Expansion. There are three such devices scattered throughout the "normal" universe, each one guarded by an all-but-invincible Transformer that is under Jhiaxus' remote command. These guardians are Thunderwing, Sixshot, and Monstructor.

Fiction

2005 IDW continuity

Arcee, in her rampage to destroy all things Jhiaxus, attacked a facility that housed the Nega-Core. Ultra Magnus managed to incapacitate her before she destroyed it, while Jhiaxus's drones informed him that the device was undamaged. Spotlight: Arcee

Once the Nega-Core was complete, Nemesis Prime dispatched Cyclonus on a mission to activate the Nega-Core on Corata-Vaz and its guardian in preparation for the Expansion. Cyclonus allowed himself to become distracted by the Ark-12 and barely reached his target in time. He initially planned to ignore his secondary objective, out of a personal distaste for what Thunderwing had done to Cybertron, but was ultimately forced to activate the guardian when Ultra Magnus and the Ark-12 crew followed his trail to the Nega-Core's hiding place. Spotlight: Cyclonus

The two other Nega-Cores were on the planet Rotan and the fourth moon of Bhul, guarded by Monstructor and Sixshot. Spotlight: Doubledealer

In the face of such powerful guardians on Rotan and Bhul, the Autobots just calmly took over a space bridge portal, snuck in the back to remove the Nega-Cores, and then snuck out again while the Guardians weren't looking. With Thunderwing disabled by an axis-override cradle, all three Nega-Cores were able to be dumped into the Dead Universe by Cloudburst's unit, where they exploded and sealed off the Benzuli Expanse anomaly. Spotlight: Sideswipe

Ask Sideways

After hijacking Vector Prime's Axiom Nexus News column, known liar Sideways claimed that the Dark Spark was the result of him forgetting to shut down the Nega-Core he was guarding. Ask Sideways, 2015/07/01