Undertale spoilers
@kara_dreamer@witches.town ahh, yeah, though NOT in the genocide route as it turns out. It's in the true pacifist run, via text logs. This will get long, so bear with me , please. ^^;
Turns out when a monster is killed, because their SOUL is their entire being , suffused with magic (hence why every monster shows up as a greyscale image in the battlescreen, whilst you are just a heart), their soul is destroyed and cannot persist at all, except in the case of boss monsters.