When Tirathon Saltheril throwing the fel mortar they are not marked in the ground where the flames will fall and summons too many moobs of the furious flames (they must be invoked 2, one of each fire).
Official server mythic 8: https://www.youtube.com/watch?v=WA5GuqRoisM&feature=youtu.be&t=231
image firestorm mythic 6 : https://imgur.com/a/A7OB8