After a talented teenager foretold to be their future-supervillain is kicked out of the “planet of the heroes”, he discovers that the prophecies are infact a ploy by the great seer to usurp the throne and bring back the age of dark magic.

Take 2

"After the prophecy robs him of his superpowers, an exiled hero discovers the oracle, conspiring to bring back the dark age"

I think it's pretty awesome and a cool concept, but there's usually this "now he must do so-and-so", there's always a an objective that is to be mentioned in the logline. Also try and make it short. For example, "planet of the heroes"? You do know it's name right? Try using it, or say "home planet" or whatever. Just little nitty-gritties to work on, to make it snappier, but I think it's a fantastic idea!

Your logline ends when the story begins, you should try writing?a forward logline which tells us what your hero must do.
Here would be an example of a forward logine, yours would be different of course.
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"When he is banished from the planet of Heroes, an outcast teen must rally a misfit group of villains to take down an evil seer and save his former homeworld."