In a mysterious manner.

Of unknown origin. / Having unknown qualities. / Difficult to understand. / Deliberately evasive or enigmatic.

Something secret or unexplainable; an unknown. / Someone or something with an obscure or puzzling nature. / (obsolete) A secret or mystical meaning.

plural of mystery

(Ancient Greece, Ancient Rome, religion) a number of secret societies or cults

The quality of being mysterious.

(obsolete) mysterious / (religion) relating to mysteries

plural of mysteriousness

(poetic) Full of mystery; mysterious.

A bag whose contents are unknown. / A surprise package randomly picked, a lucky dip. / A non-specific mixed lot of a product at the supplier's choice.