Her notion is that an order grew corrupt, and forfeited its powers, and when the evil they were fighting surged back, the one knight who could still summon them stuck out like a sore thumb.
Now, no matter what you do with that -- the muse is going with charges of warlockry against him -- you do have to invent an ancient evil to go away and surge back. Nasty enough to have special powers necessary against it.
Continuous evils and evils that are new are so much easier to handle. Can the hero even destroy the evil this time? If so, why didn't they do it last time? What changed? How many times did it return before? Will it return again? It kinda puts a limit to the happy ending if the hero knows that it's only for a time. If it was to have time to prepare -- well, knowing human nature, I suspect that even if you are not dealing with a corrupt order, no one really investigated how to do the job right. Even if the time frame was short, there were probably more urgent things on the plate.
And its nature has to be such that it's plausible that stuffing it away would not defang it.