Two things:
-
The skill itself is called “Rebuke” and not Fireball if you look in the enhanced tree.
https://i.gyazo.com/22cb23f4d057eab686425e72907adc96.png -
If you skill “Unchained Fire”,instead of costing 0 Mana, Fireball suddenly costs 297 mana. That does not change even if you skill Flamethrower after it.
https://i.gyazo.com/2501fdbb63d1486b7f28d5497dd27267.png