I may have solved it, because it just happened to me! In a multiplayer game. And it was my heroic prophet....
It's a Demonbred, in your case and in my case. Important note: Demonbreds can fly. That means, with high fatigue, they can warp around the map and end up unconsious when they land, and die instantly when attacked.
So, I watched a long battle that went nicely... and was suprised to see that my Demonbred had died! I watched 3 times, and finally realized: He was set to "cast spells", but regardless, when all the enemy routed, and there were only a couple enemy units left at the far right, he warped to the far right edge of the battlefield and ended there with 113 fatigue, and was slaughtered by a routing archer.
So, check the frame before he dies, and see if he warped to the far right end of the battlefield, and was killed by a routing enemy.