I assume this is where the Headless Horseman came from...
https://steamcommunity.com/sharedfiles/filedetails/?id=2882795116

The woman in rank 3, Magician (mod class), has an ability that summons a floating rock in the enemy ranks (for various benefits). When used on the Horseman fight, this has unintended consequence. He was taken to half health while the floating rock was in play, and dismounted... into nothingness. He did not spawn on the battlefield.

The horse died easily, and the floating rock despawned. The fight did not end.

I was stuck there, bashing away at the rubble for over 70 turns, at ~4 hps of damage per turn, total, because nobody had any armor-pierce. Fortunately, when the rubble died, the game auto-ended the fight and I got a pile of pun for a reward, so not all bad. ;)

Still, if it's possible, might want to code in a check to see if there is space for both horse and rider on the battlefield, and if not, force a despawn of rubble or horse (modder's preference) so the rider (the actual danger of this fight) can spawn properly.