For "stranger":
* Alien: Implies something completely foreign and unfamiliar.
* Outsider: Emphasizes a lack of belonging or connection.
* Unknown entity: Focuses on the lack of identification.
* Unidentified: Similar to "unknown entity," but emphasizes the failure to recognize or classify.
* Anonymous: Highlights the absence of a name or identity.
* Elusive: Suggests the difficulty in finding or identifying.
For "unknown":
* Uncertain: Indicates doubt or lack of clarity.
* Unfathomable: Emphasizes the difficulty of understanding.
* Enigmatic: Suggests a puzzling or mysterious quality.
* Obscure: Highlights a lack of visibility or recognition.
* Inscrutable: Implies an inability to comprehend or decipher.
For "mystery":
* Puzzle: Focuses on the need for solving or understanding.
* Riddle: Emphasizes the need to decipher a secret or solution.
* Conundrum: Suggests a complex or perplexing problem.
* Secret: Highlights something hidden or concealed.
* Intrigue: Emphasizes the curiosity and interest generated by the mystery.
* Aberration: Implies something deviating from the norm or expected.
Combinations:
* Elusive mystery: A mystery that is hard to uncover or understand.
* Inscrutable stranger: A stranger whose motives and intentions are impossible to comprehend.
* Mysterious unknown: Something unknown that evokes a sense of mystery and intrigue.
The best word or phrase to use will depend on the specific context and the tone you want to convey.