I've invented the supernatural 20 spell that can only be used once between long rests. On an important roll you can instead cast your Supernatural 20 spell and automatically roll a nat 20.
It's also a meta spell in that the spell affects the game mechanics. So maybe it should be thought of as a perfect attack spell but I love the name Supernatural 20.
So when you cast the Supernatural 20 spell the attack affected automatically results in a perfect attack exactly as if the atteacker had rolled a nat 20.