It was either that Prince Harry had some low-key grievances against Harold or his mother had incited him too much because by the time Damian led him out and went to find Ivan, Harold had shown up in the training range, and things got crazy.
Damian didn't exactly hate Harold since he wasn't from the queen; neither did he give anyone trouble and just lived quietly. But since he had gotten involved, he could not just stop now.
So when both Harold and Harry were unconscious, he did it.
He blew a poisonous powder at Harry. He didn't want to take any chances and have Harry remember what he had done.
But Tyra had seen him.
And it was from that moment on, the brainwashing started. It was all to bring her to his side. And Tyra, a weak little girl with an inferiority complex, had fallen for it.