NIKOLAI almost forgot that Mona was truly fearsome when mad.
He was overjoyed by the fact that his whole family was finally complete. Thus, he forgot that Mona had plenty of reasons to hate him. She initially asked him to tell her the terrible things that he put their children through in the past, but he genuinely didn't know where to begin, so he kept his mouth shut while thinking.
That was when Mona lost her temper. Instead of waiting for him to explain, she just used a skill on him.
It was the skill that brought him to his knees literally while maintaining eye contact with Mona. Her light-blue eyes glowed beautifully. But although it was a mesmerizing sight, her eyes were actually "reading" his memories through his eyes.
In short, Mona was looking into his memories– memories related to their children.