I drew the image of the keep some time ago, just as a small sketch in a notebook. However I found myself returning to it as I took a liking to the idea of a small keep in the middle of a deep and dark forest. Just recently, I decided to draw the maps of the keep, should someone want to use it as an adventure location. At some point, I might just flesh it out a bit more and add it to an adventure.
I’m not a great adventure writer, and certainly not in English (it’s not my native language) but I do try to give my adventure sites descriptive names, and I hope they can provide some substance to the place.
In this case, I’m picturing an elderly ronin-type knight, his master killed or disgraced. The knight is living out his last years as a recluse, not participating in whatever game the outside world is playing. Perhaps the player characters need to convince him to take part on one final quest. Perhaps he is a keeper of great secrets and has information the adventurers need. In any case, just finding the Knight Hermit could very well be an adventure in itself.
As DM, you could act out the Knight Hermit a bit like Obi-Wan Kenobi from the early Star Wars movies. If you want a more corny character, perhaps slightly mad from the isolation, you could base him on Yoda.