The following article contains spoilers.
Please read at your own risk.
Prince Thomas is the son of the current king of Adria and the only child of Princess Ann.
His mother is secretly arranging a marriage between him and Grace Blakely in order to put Princess Amelia's bloodline back on the throne of Adria. Prince Thomas helped Grace find the remnants of the segment of the royal family that died during the coup, and went to tell Dominic what they discovered. He shot his mother, Ann, to stop her from murdering Grace, and destroying the evidence that Princess Amelia did not die with the rest of the royal family during the coup.