There of course cant be a lord of the rings mod without the one ring.
Maybe it could be put in the game as an item for generals to posses? When they have it they cant die of old age and it could boast their abilities greatly, and when killed it will be transfered to the general that lead the attack.
The ring could also be transfered between your generals.
The possesion of the ring could cause a lot of effects.
the general could get some sinister traits over the years, but their hitpoints, the troop morale and movement speed could be improved. It would also cause opposing armies to fear this general more, causing them to flee sooner.
Loyalty could be decreased, which would make sense since it corrupts.