so I saw this monster with level 106

It was probably spawned for the kamikui killing halloween event. That is how it looks when dead, so it will probably be there until maint on tuesday morning.

I tried to attack it, but it didn't take damage xD

It shouldn't be taking damage cause it's dead~

Unlike what is practically every other monster in the game, Satan does not actually despawn when he dies. Instead, he just sits there, as you've already seen =P