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.
Legendary Resistance (3/Day, or 4/Day in Lair). If the sphinx fails a saving throw, it can choose to succeed instead.
Spellcasting. The sphinx casts one of the following spells, requiring no Material components and using Wisdom as the spellcasting ability (spell save DC 20):
At will: Detect Evil and Good, Thaumaturgy
1/day each: Detect Magic, Dispel Magic, Greater Restoration, Heroes' Feast, Zone of Truth
Multiattack. The sphinx makes two Claw attacks and uses Roar.
Claw. Melee Attack Roll: +12, reach 5 ft. Hit:20 (4d6 + 6) Slashing damage.
Roar (3/Day). The sphinx emits a magical roar. Whenever it roars, the roar has a different effect, as detailed below (the sequence resets when it takes a Long Rest):
Arcane Prowl. The sphinx can teleport up to 30 feet to an unoccupied space it can see, and it makes one Claw attack.
Weight of Years. Constitution Saving Throw: DC 16, one creature the sphinx can see within 120 feet. Failure: The target gains 1 Exhaustion level. While the target has any Exhaustion levels, it appears 3d10 years older. Failure or Success: The sphinx can't take this action again until the start of its next turn.
Collectors and Keepers of Secrets