The Time Celestial is a villian and member of the alien race known as the Celestials,
Cronus boss

The Time Celestial's first appearance from page 19 of The X-Men Genesis

and one of the main antagonists of the Doctor Who/Marvel universe. He was the one behind the crash-landing of the Shi'ar ship, manipulating events in order for them to form the team known as the Vortex Guardians. He is one of the few entities that know the Doctor's true name, something he describes as 'Having a lot of vowels'. Being a Celestial, he is capable of appearing like anything, but he often assumes the form of a huge, multi-headed serpent, much like the Hydra of Greek mythology (no reference to the evil organization HYDRA). The Doctor, Melody Pond and the supervillain known as Magneto encountered this entity on the planet Suraxon during the Mutant Revolution. Manifesting in a massive valley of mist, he proved vulnerable to Magneto's attacks but soon overpowered him. The Doctor eventually outsmarted the Time Celestial by taunting it with the fact that destroying the Doctor has no purpose, for 'the universe will never forget me'. Magneto opens a space warp and leaves, while the Time Celestial releases the Doctor and Melody. Before the Doctor leaves, the Time Celestial telepathically tells him: 'The universe may always remember you, Doctor, but it shall do so in Silence.'