Invoker is a hero that was once in the game, but is unlike any other. He has no stat bonus ability, has 3 different "spells" with seven levels each, and four levels of ultimate (that he begins receiving at five).
His three core abilities - Quas, Wex, and Extort - act in combinations of three. If you click Quas twice and Wex once you will get two Quas orbs and one Wex orb around you. These orbs provide very minor stat bonuses themselves. But his real ability occurs when you click his ultimate, Invoke. He then gains a spell based on the three components you have activated. This gives him ten different spells he may choose from. He is a hard hero to learn at first, but an interesting one, although he has been toned down from his previous version.