[Gandalf Parker]

There are many ways to steal commanders. Various nations have units that can seduce, some can summon units that seduce. There are also spells that can be used in combat such as Charm or Hellbind Heart.

There are also spells to steal commanders from their province which makes it easier. Summon them to a province with a lab and gang up on him with units that can do Charm or Hellbind Heart spells. Wind Ride or Winged Monkeys can steal a commander away from his army.

Even if you just kill steal them and kill them it still can break up a large army into smaller lagging ones that are easier to kill off. And if you arent able to make make it useful by getting him to one of your provinces, then an interesting twist is an assassin that can do charm or hellbind. In a missed assassination, you can make it yours. Then a combat will follow in that province with the charmed commander fighting his old army buddies. If no other commanders are there you can watch the guy rout his old army for you.