Master Units are the 'Epic Units' in Rise of Legends. Boasting an impressive amount of firepower and deployment prices, Master Units are powerful in the right hands yet only one can be deploy to serve their owner.
They are treated as 'Heroes', hinted by their appearence in the Hero Bar, amongst the other heroes/heroines. They too have thier own abilities to use against their foes better still can be upgraded into a more powerful version, providing the nation can afford it. Due to their power and financial difficulty, they are a late-game unit after the nation has acquired enough research to unlock them.
Take heed though. Even if they are extremely powerful, if left unchecked, a fearsome army or worse still, another Master Unit, can bring about their fate.