I know I saw this somewhere. It was a bloodline, or mystery, or...SOMEthing.
Trying to make a caster with a slashing weapon magical theme, a la
<a href="http://goblinscomic.org">Thunt's</a> IMEs.
Any direction is appreciated.
I don't know anything that will generically change any type of spell damage to slashing damage.
If you home brewed one it should probably cost the same as Elemental Spell metamagic, and would allow roughly the same thing. Elemental Spell is used to bypass immunity or resistances, which would be the main way you could use this ability (even if you want it for just for thematic purposes).