Option 1: Pick whatever you want and explain it as your heritage finally expressing itself having been passively there all along.
Option 2: Have your new sorcerer powers be inflicted upon you after something infects you. You can pick a recent event in the game that makes sense or work out something with the DM.
I played a similar-ish designed PC once upon a time: 5 Levels of GloomStalker, then 1 Level of Cleric of Order, then 5 Levels of Divine Soul, then 4 Levels of Assassin, then 3 levels of Battlemaster, than 2 more levels of Sorcerer. She was a Tiefling that believed, fiercely, that Asmodeus was misunderstood. Everything that Asmodeus did was necessary to fight the Blood War - and that the rest of the universe was really to blame for not supporting the efforts of the Devils to keep the Demons from winning the Blood War (which would end the universe). She was an Archer and this storyline was a backdrop until I added that level of Cleric of Order. However, she was less "organized religion" and more "this is in my blood" as a Tiefling, so I went with Divine Soul Sorcerer from there. She developed a role as an assassin so she switched from the sorcerer elements to a little Assassin and then Battlemaster benefits before catching a couple more levels of sorcerer at the tail end of her existence.
When I tell people I played this PC who had 5 classes they ask about why I built it that way. While it was a very efficient PC at all times, the reason was because the story lead me there. I made choices that followed the storyline of the campaign.
