He appears as a large ring consisting of pieces of his face, as well as enlarged parts of his blasters and tilted versions of the symbol on Papyrus's uniform. He appears to have lumps growing off of him, and several tenacles and arms grow off of him. Papyrus' waist can be seen, as well as Sans' shorts. Inside the ring is a large mouth with a big, toothy grin with some floating teeth. Below it are four sets of four teeth, and above it are two photo-negative mouths with what seems to be an inverted version of Sans' overworld sprite's head, as well as an orb above it that turns red when angered. One of the lumps has Papyrus' eye on it.
Sanstrosity has the ability of Sans, but instead of rising bones, they come from above, in a different shape.
His Gaster Blasters are... reversed, they shoot blue, weird lasers, and stand and move for 3 seconds, the weird thing about them is it comes from their head.