Undertale spoilers (and Charles Williams's _Descent into Hell_)
@kara_dreamer@witches.town I don't think there is any difference other than the endings on a post-genocide pacifist run. Basically to get any "soulless" as they're called runs, first you gotta do a genocide run, then relaunch the game and wait 10 minutes on a pitch black screen. Chara then reminds you that you destroyed the world, and asks if you think you're above the consequences. Selecting yes causes them to reply "Exactly"