Over the course of a long rest, your mind wanders and becomes open to terrible possibilities, allowing you to cast this spell and forgo three spells that you know or have memorized, allowing you to access a single spell from any other spell list for the duration (of a level you are able to cast, and you must have an open spell slot of that level available). The spell slot used for this spell is still consumed after this long rest and only returns on the one after that.