You have trained as an assassin that specializes in the use of magic.  As such, you have the ability to perform a magic "sneak attack" with spells.  You gain the following benefit:

• Once per turn, you can deal an extra 1d6 damage to one creature you hit with a spell attack if you have advantage on the attack roll. 
• The damage type is the same as the spell. 
• If the spell deals more than one damage type, you choose which type the extra damage is. 
• This extra damages increases to 2d6 when you reach level 10.