This AU’s name is Spinswap, check SwapSpin.
Sans, the brother of Papyrus, who died of an illness, is in the place of Asriel. He still has slight points of Sans' personality, but keeps some of Canon!Swap Sans' and Asriels. There is no official take, as it's a fan owned AU, meaning the whole community may interpret Sans how they please.
As said before, he has no official usage, so the attacks may vary, though most ideas usually include both Sans' with an Asriel twist. For instance, maybe instead of stars, it would be a bone that explodes into smaller bones, so on and so forth.
Yet again, depends on who's version it is. It would most likely be the same as Asriel, making him remember the past, getting him into a certain emotional state.
Usually (Again, changed depending on the version) is in a striped jacket or striped shirt with blue and yellow, referencing his "Bad Time" eye in Undertale.
His god form has way more designs. Most usually have Sans' head in Asriel robes and one yellow eye and another blue eye, yet another reference towards his "Bad Time" eye.
No canon theme, but here's a few.
This is a community owned AU.