The Banishing Book one of the TOLK series
Long before this middle age of Wizardry the Fey and creatures of the Elder days shared this world with Elves and men. The depredations of demon kind and fell creatures made life a hazard and a tenuous thing.
Shamans, Wizards, Priests, Witches, Seers and Seeresses and anyone else with even a hint of magic were called to a great meeting. It was decided to attempt a Banishment of all the fell creatures from the Elder days. A great labor was made to construct this massive spell. There under a full moon at a most auspicious moment it was initiated and released.
Undertaken in desperation at a time when the refinement of magic was in its infancy it was reliant on sheer power rather than finesse. It worked, but the Fey were dragged with it.
Now the banishing is failing and both the Fell and the Fey will return