Each turn, he adds up to 1 DON from his deck, then attaches up to 4 rested DON to a single Character
Not only can you play a cost 6 (or less) Song for free when you play him, but he also counts as a 6 cost character for playing other Song cards too, thanks to his Singer 6 keyword
Playing the Overlord on three will put you at four lands that turn, and if you have another land drop on your fourth turn, you can cast another Overlord for the full five mana, as a creature this time, while further ramping you to six lands
The main reason it was banned wasnt because its overtly oppressive, but because its so good as the commander of just about any strategy that it actually stifles creativity and diversity, and diluted new deck ideas, and fielded out more synergistic commanders in their own decks
You're spending cards to wall off power