Is he the Void?
The Soul of Padomay?
The Tom Bombadil of TES?
Does he even exist?

He is the name given to the powerful force also known as Padomay. He is the void. Some think he's a conscious god, others just simply a force of nature. There's even a theory suggesting Sithis is actually Lorkhan, and not Padomay because of the statue he has in oblivion that is missing a heart.

Sithis is the beginning of all. He is not the soul of Padomay, he is Padomay. He resides in the Void, and is the physical representation of chaos. He is the opposite of Anu, the father of the et'Ada. He is responsible for all of creation. Without Sithis, there would be nothing. Not even the Void.

Keep calm and hail Sithis!