Just a simple question, it is clear he is the plain dark and cold emptiness of space the void. But can he assume humanoid form like he is shown in some art and statues? . If so he would make an excellent antagonist in a future game in my opinion.

That Sithis doesn't exist. The Sithis you're thinking of is actually Lorkhan, and Lorkhan is too dead to take on a physical form on his own the form of a living mortal, but he can be reincarnated in a shezzarine.

well it does make sense since the void is nothing sithis is nothing. But still. Lorkhan is the son of sithis yes? and is the enemy of akatosh his cousin son of Anu
and both are actually their father?.

Sithis is nothing. He only exists because he doesn't.

Sithis is a primordial being of entropy, chaos, and darkness. A better quesion is if Sithis is actually aware of itself, and if so, gives a damn about Mundus. For all we know, Sithis and Anu are so far above what is known in TES that, to them, every conflct on Mundus is similar to that of two ant colonies waging war. Yeah, they're doing it... but we don't really care if one side or the other wins.

Here we go.