Grab your headphones/speakers, plug 'em in/turn 'em on, and press the green flag! Watch and listen! MASSIVE SPOILER ALERT! Not recommended to watch unless you've finished the True Pacifist Route/Watched a playthrough of it. EPILEPSY WARNING! At the very end.
DISCLAIMER: I do NOT own Undertale in any shape or form. I did not develop the game in any shape or form. I have completed True Pacifist 4 times, and am doing my second Genocide run. ( I quit on the first one due to Sans... I'm stuck on him again XD )