Frau already married before.
He has a son and his wife was a maid who worked in their Duchy.
He was disowned for having relations with a commoner so he had to raise his son on his own. His wife eventually died.
Fast forward to when he met Poliana. He was already scheming behind her back. He proposed to her. Honestly telling her that he wanted her money and power which surprised Poliana but she appreciated his honesty and genuinely started to like him. He, however, never revealed the existence of his son.
He planned to marry Poliana and then tell her that he already has a son and make her adopt that boy and make him her heir. The next Marquis of Winter. He wanted his son to be a noble like him before he fell in love and was disowned.
He planned to ambush Poliana with the truth after the marriage and so she would be forced to accept the child and eventually give her noble name and house to them.
Which is really sick, mind you. Poliana already had her period and was imagining herself raising a family with him. She was so excited. ╥﹏╥
Everything fell through when she realized his scheme (Donau and Hau were the first to know and then the Emperor and all the soldiers Poliana fought with during the war did a thorough investigation and found out too. THEY WANTED TO KILL HIM. But they decided that it was best for Poliana to take care of this herself.)

Why do I want her to end up with the healer's apprentice?(⌒▽⌒')