Spellwarp

You know how to warp magic to exploit a foe's distraction. Once per turn, you can deal extra damage to one creature you hit with a spell attack roll if you have Advantage on the roll.

You don't need Advantage on the attack roll if at least one of your allies is within 5 feet of the target, the ally doesn't have the incapacitated condition, and you don't have Disadvantage on the attack roll.

The extra damage begins as 1 point and increases by one at each odd-numbered rogue level (3rd, 5th, 7th, ect.) 

This replaces Sneak Attack. This functions with Cunning Strike and Devious Strikes (reducing the extra damage by 1 per 1d6 it is normally reduced by). The DC of saves made against an effect from Cunning Strike or Devious Strike is based on the ability score used to cast the spell it is applied to. 

(This is intended as an Optional Class Feature replacement for the rogue's Sneak Attack)