The phrase "a mysterious box" is correct and usable in written English.
You can use it to describe an object that is intriguing or enigmatic, often in a narrative or descriptive context.
Example: "As she opened the attic door, her eyes fell upon a mysterious box covered in dust, sparking her curiosity about its contents."
Alternatives: "an enigmatic container" or "a puzzling box".
Set in a post-apocalyptic future, this story of five teenagers is predicated on a mysterious box thrown up by 18th-century wreckers.
Hunched over a mysterious box of tricks, Carl Michael von Hausswolff squints at a range of dials, a cigarette burning down towards his lips.
Two young kids find a mysterious box that gives them strange powers and could be the key to saving the future of humanity.
It sees "pleasure seeker" Frank Cotton stumble upon a mysterious box in pursuit of the ultimate sensory experience, only to release monsters including Pinhead.
But that is only the beginning of this Japanese fable, also involving a mysterious box and a promise; it will unfold when the Saeko Ichinohe Dance Company performs "The Fisherman and the Tortoise".
Family Tree will feature 30-year-old Tom Chadwick, played by O'Dowd, who loses his job and his girlfriend only to stumble upon a mysterious box of belongings from a great aunt that he never met.
It was staged by a band of people called the Yihequan ("Righteous and Harmonious Fists"), who believed that a mysterious boxing art rendered them invulnerable to harm.
The company has lifted the decorative kerchief that's been draped over a large, mysterious box to reveal a new 23-inch 16:9 HD monitor and a small form factor PC known as the Aspire X1700.
Perhaps as a result, the mysterious box has been described as a classic "MacGuffin" - or an arbitrary plot device of no real significance ("The 39 steps are... .. Bang! "Aaargh!") - and as a surrealist device like the boxes in Luis Buñuel's Un Chien Andalou and Belle de Jour, and the briefcase in Quentin Tarantino's Pulp Fiction.
"It's a mysterious black box".
But things take a turn for the strange when Leo finds a mysterious purple box.
