After Optimus Prime, Megatron, Arcee, Bumblebee, and Bulkhead GroundBridged into Unicron, they were initially unaware that they had been spotted by anti-bots. It wasn't until they were crossing a vast space that the creatures attacked in masses. They proved difficult to overcome, not because they were especially powerful, but by sheer virtue of there being lots and lots of them. After Arcee became separated from the others, she rode an anti-bot to rejoin them. Optimus and Megatron entered Unicron's spark chamber, leaving the other three outside to hold off the anti-bodies. Once Unicron's spark was extinguished, the remaining anti-bots dropped out of the air like bricks.