"A patient fey is trying to derange a disciplined constable in a crumbling tower."
That's more than enough to riff off of; from here, start asking questions of the sentence to build outward:
Why is the fey trying to drive this constable insane--did they do something against the fey long ago since the fey has been 'patient'?
Why is the goal to drive the constable insane--is their disciplined nature what they value most about themselves and the fey wants to take that away?
What is the tower? Is it a guard tower now besieged by fey incursions? A pagan site the constable once defiled in some way? A trap that the constable has been lured into in a remote location?
Feel it out, sketch it down, fill it in. You're ready to go.
Note: You can swap columns two and five or columns one, four, and six for even more options.