What is the name of the character who warned the others about the Fallen's return?
In 17,000 B.C., the Seven Primes travel across the universe to create Energon with star-absorbing machines called Sun Harvesters, which destroy stars in order to collect their raw material. One of them defies the rule to never to destroy a planet with life by establishing a Sun Harvester on Earth, earning him the name "The Fallen". The Fallen is confronted by the other Primes, who imprison him before he can harvest the Sun using the Matrix of Leadership. The rest of the Primes then sacrifice themselves to hide the Matrix in an unknown location.
In the present, two years after the events of the first film, the Autobots and the U.S. military have formed the Non-biological Extraterrestrial Species Treaty, a classified international task force used to eliminate the surviving Decepticons. In Shanghai, the Autobots intercept two Decepticons, Demolisher and Sideways. Sideways is killed by the Autobot Sideswipe, while Optimus Prime kills Demolisher, but not before he warns them of the Fallen's return. At NEST's secure headquarters in Diego Garcia, National Security Adviser Theodore Galloway concludes that since Megatron's corpse is at the bottom of the Laurentian Abyss and the last-known AllSpark shard is secured in the base, the Decepticons must still be on Earth solely to hunt the Autobots. The Decepticon Soundwave hacks into a military satellite, overhears this information, and sends Ravage to retrieve the shard.
Sideways