The brightest of flames, corrupted by the foulest of shadows.
Toro was once a Ta-Matoran living on Artakha. The island's ruler saw potential in Toro and transformed him into a toa. As Artakha was a very safe place, Toro was not needed as a guardian and instead devoted much of his time to understanding the nature of his powers and refining them into his characteristic blue flames. He was even able to access some Spirit elemental powers.
He was later sent to Korrothis, home of the Elemental Council, to train further and to train other toa of fire alongside the island's resident Archons of Fire.
When Teridax took over the Matoran Universe, Ta-Nui Delnok and his archons violently opposed the rest of the Elemental Council's decision to flee the Matoran Universe, ultimately leading to their being marooned on the Northern Continent as Korrothis blasted a hole in the Great Spirit Robot's chest, destabilising the universe's balance of elements as it did so.
Delnok and his archons remained and fought Teridax's forces, and for a while they kept their ground - until Toro was captured.
A prodigy with his fire powers, Toro had little combat experience and was quickly separated from the group. His raw power was perfect for a new plan Teridax was putting into motion.
Teridax took Toro and combined his body with Antidermis and Rahi parts to create Cosrahk - the King Rahkshi, far superior to any others he had created. Stripped of his willpower, Cosrahk was given purpose in commanding its own rahkshi army. Its first test was to destroy its former master and team-mates, which it did with devastating ease.
Cosrahk remained commander of Teridax's forces for five years. Occasionally, flashes of his old life would come to haunt him, but Teridax was swift to silence those memories when they did. When the time came to launch the attack on Bara-Magna, Cosrahk led his horde of Rahkshi, Rahi and other monstrosities against the allied forces of the Amu-Skrall, Vampirik, Korrokoa and Glatorian.
After Teridax was defeated, there was nothing to stop Toro's memories flooding back. He retreated into the ruins of the Matoran Universe, seeking solace. He remained this way until Gloriana found him and persuaded him to join her cause. Glossing over the rahk part of his name, she gave him a nickname: Cossy.