That's probably Damian - you will need to play Divine Divinity and / or Divinity2: The Dragon Knight Saga to find out who he is. It's also hinted at during the game who his is as well but D2:DKS explains it all if you have played that.
He's the demon child who Lucian was meant to kill but ended up adopting instead at the end of Divine Divinity. He formed the Black Ring who are not totally defeated at the end of Original Sin 2. The Dragon Knight Saga is set after OS:2 and centers around the player character stopping him and the black ring. So it really is Lucian's fault why the events of Original Sin2 and Dragon Knight Saga took place. To be fair who would want to kill a newly born baby even though it does contain a demon?