I know what the tune is. Last night as I was with my horse and unicorn, the unicorn blessed my horse. This turned him into a unicorn! He was an elder, (just had his birthday), and I don't think unicorns are supposed to become elders. Anyway, once he aged and was blessed, the song played and along came Grim and took away my beautiful new unicorn/horse. So I believe the song is for when a unicorn dies. Unicorns are not supposed to die, therefor the very sad sound of the song.
When Grim came for him, no big fanfare. No big whahahahaha. He just stroked my uni-horse, hugged him, mounted him and flew away with a mixture of black smoke and sparkles. It was as if Grim was asking him if it was ok for him to be Grimmie's new ride, because before Grim mounted him my uni-horse nodded yes. I cried for hours.