He got a music made by the beautiful Grass: https://youtu.be/0k9v3LrxB_U?si=pNVL8xdZ1jhdjpM8

Here’s the tiny bit of lore he has (he won’t have more): every time you die in any sans fight (from au to at) you have a tiny chance of respawning in in a red lit corridor and face this sans.

He's the judge and avenger of everyone you killed (au and at included) and his fight difficulty depends on how much people and worlds you destroyed.