Lord Bevin was still unsure. But he feigned surrender and joined Su Wan's side according to the plan.
His hero, the Red-Eyed Black Dragon, was unable to hold it in any longer and almost blurted out everything. He wasn't deliberately planning to keep his Lord in the dark.
However, the Light-Types had placed a mark on them. They had done it, theoretically, for their safety. The mark would allow them to help if their plan failed and if Lord Bevin was attacked. The Light-Types could hear every conversation between Lord Bevin and his hero. This was the reason why the Red-Eyed Black Dragon couldn't tell his Lord the truth now. The Red-Eyed Black Dragon knew that the mark was a listening device to spy on them.
The Red-Eyed Black Dragon couldn't tell Lord Bevin the truth, but maybe he could signal or give him hints through other means. But he didn't. He was afraid that his Lord would panic after knowing the truth and the Light-types would find out everything.