How to get this:
On your 1337th genocide run at 3:00 AM, At any point on the Sans fight, Play on the Piano the 19th key, then the 1st key, the 14th key, and the 19th key. (in alphabet its spells SANS) It will skip to the end. this will happen.
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https://www.youtube.com/watch?v=fp7aJBdyVDU