
Players is skilled with the dagger and may add a d8( plus an additional d8 every 2 levels) to your d6 for sneak attack but must be using a dagger and no other weapons while attacking additonally player must be behind target at all times for this to take effect.

If it replaced the d6 with a d8 and you had a good story reason for it, I could make an argument for this. Simply adding d8's to the d6's the rogue already gets is worth an ASI and two epic boons.