The offer of power was tempting, but Markus had seen someone he knew let the darkness in, and he certainly hadn't seemed in control. Maybe some day, but today that dark portal in the sky only looked like the black hole he had narrowly avoided back in his own world. The black hole, with its keyhole.  Markus thought back to that, when he had unlocked it.  That man, Leon Nero had called him, said that his keyblade could unseal any lock. 

"Nero," Markus called across to him, "Akira. All of you, do as I do," he commanded, as he lifted his keyblade to point at the heart. He imagined it as the keyhole, and willed his keyblade to emit that same beam of light. He hoped the others would heed his orders, as he didn't think he had enough power on his own.