Red

Arthur concentrated, eventually managing to make his lower half a mist of light while maintaining a corporeal upper body. Only then did he sigh a breath of relief. 

His ability was far too difficult to control. It wasn't because its actual activation was difficult. That alone was as easy as breathing. It already felt as though it was a part of him. 

However, there were two problems. The first was that he couldn't take any objects with him when he activated his ability. 

Well, this wasn't entirely accurate. When it came to ordinary objects, if Arthur concentrated, he could turn them into flowing lights as well. He could even take these items with him as he entered his 'flowing light' state. The issue was that he didn't have time to concentrate so much on a single task in battle.