Dracothion is the Star Drake, a being in charge of restarting realities whenever Chaos destroys it (So...who's his boss?). He is associated with the realm of Azyr. GeeDubs were probably watching Dragonheart when wondering how to justify Age of Sigmar existing at all and decided it was a very good idea.
Anyways, he is the one who found Sigmar drifting around while gripped to the core of the World-that-was, after giving him first-aid with dragon breath (yeah, Sigmar is that tough). They became good pals and worked together to kickstart the multiverse and civilization.
May or may not be Sotek. There are hints, but no official word. On the other hand, Dracothion does have celestial power similar to what's attributed to the Old Ones - especially with his spawn, Dracoths and Stardrakes, being reptilian - and it was he who helped the Lizardmen (now Seraphon) to settle in High Azyr.