Spark containment unit
When Megatron stripped the spark out of every Transformer on Cybertron, he needed somewhere to put them. The spark containment unit is built into his Grand Mal for just that purpose, and now holds almost the entire population of the planet, save Megatron, his Vehicon Generals and a few lowly Maximals.
Fiction
[edit]Beast Machines cartoon
[edit]Megatron built the containment unit to hold the sparks he took from Cybertron's population. There they stayed, hidden away, until Megatron was ready for his big moment—his grand ascension. As a horrified Optimus Primal watched, Megatron began plucking sparks from the containment unit and consuming them. The feast didn't last long, as Nightscream used his sonic scream on Megatron's spark, depolarizing it and releasing the sparks which returned to the containment unit. Sparkwar Pt. III: The Siege
After the Maximals took control of the giant head, though prevailing opinion was that they should use the sparks in the chamber to create reinforcements, Primal refused on the basis it would endanger the sparks. The containment unit therefore remained sealed while the Vehicons assaulted the head. As the ship began to crash, Rattrap used the last of the energy to erect a stasis field around the containment unit, enabling it to survive the subsequent impact. Endgame Pt. I: The Downward Spiral As the other Maximals prepared for the oncoming Vehicon assault, Optimus meditated near the containment unit and contacted the Oracle. Though the Maximals fought bravely, they were defeated and their sparks taken, bar Optimus who was driven back to the containment unit. Endgame Pt. II: When Legends Fall
He was confronted by Megatron, who added the other Maximals' sparks to the containment unit. After Optimus destroyed the Diagnostic Drones in the process of opening the spark chamber, Megatron attacked him. While the pair were busy having a city-spanning fight, more Diagnostic Drones set about repairing the damage to the control panel. They eventually finished and opened the unit, allowing the sparks to flood out so Megatron could absorb them. Endgame Pt. III: Seeds of the Future