Lies

It was a mistake. Grayson's curiosity got the best of him. He should have never let Cora go into that dungeon. Now he had been forced to compel her to make her forget about her mate, and he was pissed about it. 

It was never his intention to compel her, because it was a nasty business. Now he would have to be super careful around her in the future. 

Once someone was compelled the first time, it was far too easy for them to be compelled subsequent times by the same vampire—even accidentally. The individual in question whose mind had been altered would continue to have increased sensitivity to suggestion of any kind. 

What's worse was that the signs that she was compelled would be obvious to anyone who was aware of them. If Ambrosia or Rayth found out what he had done, they would think he was trying to use the girl for his own means. And he wasn't.