If you choose this background your character should be a yuan-ti evolver. A yuan-ti pureblood that wanted to improve upon itself but could not because of the upper classes unwillingness to complete the rituals. Because of this this pureblood stole the books from the temple containing the rituals. You were exiled because of this.

Skill Proficiencies: Skill Proficiencies: Religion, Arcana, Sleight of hand, Survival. Pick 2.

Tool Proficiencies: Tool Proficiencies:Disguise kit, Forgery kit. If you already have proficiency in either of these choose another tool. 

Languages: Languages:Infernal. 

Equipment: Equipment:Yuan-ti religious books, a set of dark common clothes with hood, Disguise and Forgery kits, 20 EP.