I haven't played it, but pathfinder, wrath of the righteous seems to solve it's lich attainment problem quite well. Liches are incredibly powerful beings. It's on par with mythic aspirations such as becoming a gold dragon.
However, Bg3 doesn't seem like it would let us become something so powerful. The evil side is centered around shadows, curses and false gods. In the trailer, we saw at one point a bunch of zombie like thralls resulting from a flawed experiment. Together with the necromancy of thay, there's enough hints towards necromancy, but it's a game of mortals vs (demi?) gods.
If you want necromancy early game, the necromancy of thay appears to unfortunately be Astarion's quest item. I assume the player could also learn a few "flawed" spells from it, since we have no idea what we're doing. (However, it currently can't be shared...). Alternatively, we could get ourselves cursed in that crypt. The necromancer quest line could give a cure or unlock the subclass.
It's certainly a way to start off with a few inferior spells or heavy maluses. Since you're cursed. Level restrictions apply.
Story wise, it can range from killing and reanimating whatever living companions you brought on accident (curse your greedy, greedy level 2 paws), to healing items harming you... How cursed is too cursed is a wide playing field.
