Inscrutable. No magic can observe the sphinx remotely or detect its thoughts without its permission. Wisdom (Insight) checks made to ascertain its intentions or sincerity are made with Disadvantage.
Magic Resistance. The sphinx has Advantage on saving throws against spells and other magical effects.
Multiattack. The sphinx makes three Claw attacks. It can replace one attack with a use of Curse of the Riddle.
Claw. Melee Attack Roll: +7, reach 5 ft. Hit: 13 (2d8 + 4) Slashing damage plus 7 (2d6) Radiant damage.
Curse of the Riddle. Intelligence Saving Throw: DC 15, one creature the sphinx can see within 60 feet. Failure: 21 (6d6) Psychic damage, and the target is cursed with a riddle. The cursed target has Disadvantage on ability checks and attack rolls. In addition, if it takes the Magic action, it must succeed on a DC 15 Intelligence saving throw or that action is wasted. The cursed target can take a Study action to make a DC 15 Intelligence check, solving the riddle and ending the curse on a success. The curse ends early if the sphinx curses another target.
Spellcasting. The sphinx casts one of the following spells, requiring no Material components and using Intelligence as the spellcasting ability (spell save DC 15):
At will: Detect Magic, Identify, Prestidigitation