Shadowcasters gain the high level ability to do sneak attack damage with attack roll spells. But so far as I can see there are none on their list. They can pick up a suitable cantrip by being an elf or tiefling, but otherwise this ability seems to be useless.
Shadowcasters get necromancy, so the following should work:
Chill touch (Cantrip)
Vampiric Touch (L3)
That said, given Shadowcaster spell progression, they'll only get vampiric touch pretty late. Vampiric touch however, may be nice if the sneak attack stacks for lifedraining, so could provide a boosted healing, assuming it works that way.
No options for L1 or L2 spells, though.
I'd forgotten Chill Touch. But probably a good idea to play a high elf and pick up Shocking Grasp as well. I'm wondering if it works with spells cast from magic items. Might make a helm of blasting actually useful.
Level 15 for Vampiric Touch.