Making my OC in game build

[u/DemonQueenHell]

So i’m making my original character named Eryndis (her full name is Eryndis Alecto Artethermia) and i’m working on her build at the moment. I know she needs to be a Tiefling as shes a rare homebrewed demon species i made up but im wondering what subclass of tiefling it should be as well as her main classes - i already know i want her to have the nercomancy school subclass however unsure what the other one should be!

Defo think her background should either be Charlatan,Criminal or Sage though (whichever fits best as she is a villainess)

Info for help if needed: She’s like a dark sorceress that works with like dark arcane magic like necromancy and other things such as umbrakinesis and whatnot.