Less distance also meant that Riku had less time to react. Though prepared for the flames he'd seen from Zuko before, the last thing he expected was to see his own darkness manipulated, increased, and hurled back at him. Eyes wide he leapt to the side, narrowly avoiding one. However, despite his own speed, the prince's attack had come too fast for him to avoid the second Dark Firaja.

The dark fireball slammed into Riku's shoulder, searing its way over his skin and sending him to the ground. He cried out in pain, but got to his feet quickly. Keep moving! The keybearer panted hard through the pain and flexed his shoulder as he moved. It was bad, but he was far from losing long-term movement. For the moment however... He snuck a glance at the wound.

A severe burn laid across his shoulder .

So he can use darkness, too... It didn't help Zuko's case.