Do you know what gravity looks like? You can feel gravity as it constantly pulls you to the ground, but you can't see it. That's my answer to what Sithis looks like.
You cannot see a force, but you can feel it. Sithis is 0, the IS-NOT, chaos, begnning and end, and change. Let me provide you what the Monomyth has to offer in the insight of Sithis
All Tamrielic religions begin the same. Man or mer, things begin with the dualism of Anu and His Other. These twin forces go by many names: Anu-Padomay, Anuiel-Sithis, Ak-El, Satak-Akel, Is-Is Not. Anuiel is the Everlasting Ineffable Light, Sithis is the Corrupting Inexpressible Action. In the middle is the Gray Maybe ('Nirn' in the Ehlnofex).
In most cultures, Anuiel is honored for his part of the interplay that creates the world, but Sithis is held in highest esteem because he's the one that causes the reaction. Sithis is thus the Original Creator, an entity who intrinsically causes change without design. Even the hist acknowledge this being.
Anuiel is also perceived of as Order, opposed to the Sithis-Chaos. Perhaps it is easier for mortals to envision change than perfect stasis, for often Anuiel is relegated to the mythic background of Sithis' fancies. In Yokudan folk-tales, which are among the most vivid in the world, Satak is only referred to a handful of times, as "the Hum"; he is a force so prevalent as to be not really there at all.
Again, you can't see a force. Though, if you were to "look" at the two halves side by side, you may see no difference between the two. And he is not the void, that's a bad assumption on the nature of Sithis. Sithis does not svck you in like a black hole. Sithis is not negative. Sithis is 0, while the counter part, Anuiel, is infinity. When you put the two together, you get variables, which formed the grey maybe
And anyone who quotes the Dark Brotherhood must consider something. You are talking with a bunch of psycho and sociopaths, and each give a completely different idea of Sithis. They are wrong to think he's some dark, goth, evil god who demands souls. In fact, Sithis probably doesn't even give a rat's ass if you kill someone or not. Just like how gravity doesn't care if you cause someone to fall on their butt.