In skirmish or multiplayer modes, every map spawns a cloaked Gollum. Find and kill him, he'll drop the One Ring. Take it back to your base to gain the ability to summon a Ring Hero for 9,000 resources. They don't even use command points! Bad guys get Sauron, and the goodie-two-shoes get Galadriel.
Thanks to Revolution Readers Sauron and beorn!