He was Sans but something gave him nightmare form which allows him to be invincible, use all of the undertale characters powers (except they would be blue and much stronger) and is very fast. This form can be destroyed if he would use too much powers thought, he can use his full power for 10 minutes, then he would need a rest
Have also fought Bendy and Nightmare mode Bendy. Killed frisk forever. His appearance is a common sans but all black with blue outline, no mouth and 2 blue glowing eyes