Warhammer fluff - The diety of Sigmar Heldenhammer?

[jus]

Sigmar unites the barbarian tribes and the realm to create the Empire. Exploits such as the prevention of complete annihilation from orcs, chaos and undead have led him through a rather successful career as bad ass king. Its said though, that one day during his 50th year, he decides to leave it all behind and take a trundle over to the mountains. Never to be seen again. Ever...

so,on what grounds does that make him ascend to bonafide godhood complete with church, followers and a holy order?

He may have been a legend, but no where near demigod primarch uberness. He seems to be venerated in the same way the dwarves deem Grungni to be a god after he wandered off alone into the WFb equivalent of the eye of terror, never to be seen again. But still thats a bit of a stretch.

If Sigmar can become join the pantheon of the old world (Ulric, Shallya, 4 ruinous powers) then can anyone else do so? Is Sigmar really the patron god of the empire, or the musings of a cult gone too far?

[Primarch]

His birth was signalled by a twin tailed comet in the sky, thats got to count for something. His priests DO gain powers, so obviously they're doing something right. Beyond that, I haven't a clue. I have never really read much of the Warhammer Fantasy fluff to be honest.