Sanstrocity is the third IT encountered in the game, and the first "Real" IT.
It's appearence consists on a smiling face in the center, with 2 small smiling mouths below and a bigger one, with even more smaller smiling mouths below. Outside of that, there's a circle of bones and eyes, with hands, feet, etc. poking out.
In the comic, You spare it by feeding it snow twice, then you use the locket.
In the fan-made unitale fight, It's pretty much the same method.
There seems to be no way to kill it, at least not in the fan made fight. It's possible, that you'd just have to run away, then fight it later in the last corridor, or at least sometime after the P A P P Y fight.