Massive planetoid Transformer that is essentially the embodiment of evil in the Transformers' universe. According to the varying story lines, Unicron exists as a sort of multi-universal being. Meaning that he may be temporarily destroyed in one reality, but is omnipresent in essence.
His most recent appearance (that I am aware of) was in a somewhat weakened state, being an extremely powerful tank-like Transformer, rather than a demonic planet omnigod.
Unicron: It is I who have summoned you here.
Megatron: No one... summons me!
Unicron: Then it pleases me to be the first.