This is quite interesting and clever - I like the idea of Luna being friends with the Sphinx, and I dig her relationship with Myst here. It’s a nice take on the prompt - the thought of Luna and Myst inventing riddles in days gone by, and Myst’s realisation that her visitor was Luna was very sweet. Also, I’m a sucker for riddles. Using the real riddle of the Sphinx, though? That’s just plain cheating, Luna, and I approve entirely.