Simple:
* The old house had a mysterious air about it.
* The detective found a mysterious clue at the crime scene.
Descriptive:
* The forest was filled with mysterious shadows and whispers.
* The woman's smile held a mysterious allure.
With Context:
* The disappearance of the ship was a mysterious event that baffled investigators.
* The ancient inscription on the stone tablet remained mysterious despite years of study.
Figurative:
* Her gaze was mysterious as the depths of the ocean.
* The future was as mysterious as a closed book.
In a question:
* What could be the mysterious reason for their sudden departure?
Remember to choose the best way to use "mysterious" based on the tone and context of your writing.