His Theme is the 90th song in the Undertale Soundtrack. It plays during the final part of the fight with Asriel Dreemurr, starting as soon as Asriel Dreemurr has a flashback of when he first found the First Human. It is a solemn, emotional song which is actually an orchestrated version of Memory.
I'm not ready for this to end.Asriel Dreemurr
I'm not ready for you to leave.
I'm not ready to say goodbye to someone like you again...
It plays at 85% speed in-game. Part B will constantly loop in-game.