On one of Frey's battles with Teridax over a world, he employed a cunning tactic: he managed to enslave and summon Exuro and pronounced him a holy warrior, scaring the whole world and saying that whereas other holy warriors are filled with holy light, this one is filled with holy fire, and proving it by allowing Exuro to use his powers. When Frey tried to challenge him, it turned out that he was much more powerful than she was. She didn't manage to defeat him and Teridax used that to strengthen his position even more by saying that she was a messenger of evil, and that even though this holy warrior would punish the non-believers, he would also protect the ones who believe with all their might.
Before his mind was taken over my Teridax, Exuro was a powerful Toa of Fire, but he was not from either Aqua Magna, or Bara Magna or even Bota Magna. He was from a world similar to them, yet very far away. His powers were constantly growing, and at some point, he began to use the power of the sun to his advantage. Very quickly, his color began to change from red to orange and after a while he became nearly powerless, but in the sun his powers were unmatched. But his great powers weren't powerful enough to protect him from Teridax's cunning influence. And so, he became one of his mindless minions.
His main attack, a powerful ray of pure sunlight. Believe me, you do NOT want to get hit by that thing: