While searching for a cure for Marcus's illness their quest lead to a long-lost Greek island called Praxidae.
While on Praxidae, they found what they thought was "Pandora's Box" and they opened it. This sent all of human kind's creativity back into the world, thus creating a second wave of newly super-powered humans.
Almost immediately Stefan turned to evil, he even killed a member of his team before they had a chance to leave the island.
Marcus escaped the island, morning what he thought was Stefan's death, only to realize his former friend had now stayed with the forces of evil and swore to destroy him.
It didn't help that Marcus married his sister either.
Powers: Being infused with the dark essence of Tarturus give Stefan power over dark magics.