Vok

The Vok were the mysterious energy-based alien creatures who had performed experiments with Energon Crystals on the Earth. As a result of these experiments, there were a number of anomalies on the Earth, including several floating landmasses, a mysterious stone formation (stone henge), and a hollowed-out moon.

Their experiments were irreperably wrecked as a result of the Beast Wars. The Vok decided to cut their losses by revealing the nature of the second moon: An immense cannon designed to destroy the planet! Optimus Primal sacrificed himself in order to destroy the weapon.

As a result of the Cannon's explosion, a Transwarp Wave was released. The wave caused a mutation in several of the Transformers (notably, Megatron, Rattrap, Cheetor and Tarantulas) turning them into Transmetals.

Later in the series, the Vok made several other attempts to wipe the Transformers, notably a second weapon which appeared on the planet (sometimes referred to as the Metal Hunter), and the abduction of the Maximals Airrazor and Tigatron.

Near the end of the Beast Wars, the Vok sent an emissary, Tigerhawk (a newly created fusion of Airrazor and Tigatron), to Earth to deal with Megatron's disruption of the timestream. This was the last (canonical) contact with the aliens.