[Graeme Dice]

Originally posted by jbw:
If an immortal pretender is attacked in a province with friendly dominion and its army is routed, it continues to fight to the death - which is good. However, the attacking army will also fight to the death, ignoring all morale.

This isn't entirely true, after around 40 turns the attackers will auto-rout. If they cannot all leave the battlefield after several more turns, all remaining units will be auto-killed.