You have about 5 minutes after the sixth trumpeteer is killed to blow the horn sequence. If you do not get it done in time you'll need to wait about 30 minutes for The Malefic Formation to respawn. The horn blowing will only work if done all by one person.
Kill the gazer Ra'Tsinis that spawns in the center for the update.
The information below has been verified with in-game data and the information on this page is correct, even if it looks stupid or makes no sense.Fact: The sequence is correct, did it two times today. To confirm just wait until the event respawns, then the horns are played in exactly this order.