Sans is becoming a SARCOPHAGUS MUMMY TRAP which basically means he a mummy in a sarcophagus whose only real attack is an insta-kill move that has to be triggered by the player... so basically he's a mummy coffin that just sort of waits around for a player to be stupid and then the mummy pops out, pulls them into the coffin, and kills them.

So this means that Sans is mostly going to be lounging around because the coffin is pretty much stationary, unless other people want to carry him places. With any luck they'll be able to fool a User with it at least once.

Appearance wise it's pretty much just Sans in a vaguely Egyptian sarcophagus wrapped in some mummy bandages that he can grapple people with. He's already a skeleton, sooooo. He can't really leave the coffin though, so haaaay.