Asriel Dreemurr is the true final boss of Undertale, coming after you've explored the bonus True Laboratory section. The following boss comes at the end of a complete Pacifist playthrough ...

Undertale's final boss, Asrield Dreemur, might just take the cake for the most tragic villains in gaming history. Used and sacrificed by his own sibling, Asriel was cursed to a wretched existence ...