Board Thread:General Discussion/@comment-27247962-20151221233541/@comment-25942629-20151222022616

The trumpets of the Apocalypse are blown during the completion of the seraph transformation and signal the end of the world. Mirai Kimizuki manifested it, and she has the most complete transformation we've seen thus far. As Yuichiro Hyakuya has lost more of his humanity while in Nagoya, he has also perceived a trumpet in his mind and has been able to come in contact with it. It clearly has some relationship to the seraphs as they lose their humanity and turn into angels.

I don't know explicitly what the trumpet will do aside from the vague description of destroying the world, but it appears to be a Very Bad Thing.