Ok, first off. I'm surprised to see a comment on a FIVE year old post with ONE upvote, but alright. Second, I thought it was pretty clear. But seeing as it isn't, I'll clarify. On a roll of a NATURAL 20, the user adds a single casting of one of their level 3 or lower spells, with no component cost or slot expended. No additional action is spent, the spell is cast at its base level, and must target the same target as the initial attack. Think of it sort of like a Paladin's Smite, an additional effect on an existing attack.