NO SPOILERS
The Third Riddle was one of the first dozen scenarios published for Pathfinder Society, and was one of the handful of "Season 0" scenarios unceremoniously retired later on. The reason this one was retired isn't a mystery: it's bloody hard! I played through it with a new character (a summoner named Jakeric with an "uplifted" bear eidolon named Grizzlebin) and a couple of others, and we were lucky to survive the very first encounter. We had to abandon the mission soon thereafter, when it became clear that proceeding any further would be suicidal. A modern group of six optimised Pathfinder PCs would probably be okay, but why the writer thought four 3.5-era PCs would be able to handle this mission is beyond me. The scenario does establish some interesting historical Osirion lore that is reflected in later setting materials, but is otherwise a very old school style mission that doesn't have a lot to recommend it today.SPOILERS!
There's a fairly elaborate (and interesting) backstory to The Third Riddle. It seems that thousands of years ago, a wizard-priest named Mektep-Han discovered the last of three known locations of the dread riddles of the insane god of magic Nethys. Solving these riddles would apparently unbind the universe, so Mektep-Han wisely had the locations hidden away. He even had his soul bound to the third location, the Ravenous Sphinx, to guard it for all eternity. Yet recently, a scholarly Pathfinder named Colm Safan uncovered the Ravenous Sphinx. The poor sod was immediately killed by a trap in the structure's entryway, but news of his discovery reached the Pathfinder Lodge in Sothis. This is where the PCs come in, with a mission to find out what happened to the missing Safan.
The briefing is told in the odd flashback style used in many Season 0 scenarios, with Act 1 starting in media res as the PCs are in a caravan on their way to the Ravenous Sphinx. They're promptly attacked by *eight* Aspis Consortium warriors. Our Level 1 PCs were pretty clearly outmatched, and at that level a few bad die rolls can become catastrophic. I liked the *idea* of the encounter (a mobile run-n-gun chase with mounted warriors attacking a fleeing wagon caravan), but despite a neat table of random events, the actual execution was problematic.
The exterior of the sphinx doesn't get much in terms of description, so reaching it may feel somewhat anti-climactic. It's essentially just a housing for a puzzle-ridden dozen crawl. The layout and room descriptions are given separately from the actual puzzles/encounters in the rooms, which can make for a lot of confusing back-and-forth for an unprepared GM (there was a similar problem with King Xeros of Old Azlant).
The entryway to the Ravenous Sphinx is a corridor in which the body of Colm Safan is clearly visible (he was obviously a much better scholar than tomb-raider!). The body is impaled on the blade of a scythe trap, and removing the body resets the trap: it's a +8 attack that does *5d8* damage (x3 crit) at Level 1! That's an automatic PC kill right there. More, the corridor has two more (though less lethal) traps beyond that one. When I played, we managed to detect the traps but then realised we had no way to disable or safely bypass them, which admittedly was a limitation on our group, and not the scenario's fault. Anyway, we decided that, since our mission was to find out what happened to Safan and we had done that, we'd take his body back to Sothis and call it a win. Hopefully, the Pathfinder Lodge could send out some professional dungeon-delvers to explore the tomb!
It was certainly wise of us not to proceed any further. The rest of the sphinx consists of three large puzzle chambers branching off from a central point. The central point contains the spirit of Mektap-Han, but he's not dangerous and there are some good role-playing opportunities there to learn more about the backstory to the scenario. As for the puzzle chambers, one features several zombies and a shadow, one features a real giant scorpion mixed with two illusory ones (a clever trick), and the third has a large viper snake. Each chamber has an additional hazard relating to the puzzle (like being burned up in one or drowned in another).
One can definitely see the vestiges of classic, 1980s-style dungeon design in The Third Riddle, and although there's certainly nothing wrong with it, I wouldn't recommend it for Level 1 PCs--they just don't have the necessary margin of error or resources to have a fair chance of success. Perhaps playing it at high subtier with Level 4 or 5 PCs would be a much better experience. Anyway, I'm still glad I got a chance to play. The Ravenous Sphinx may sit unexplored, but at last Colm Safan's next of kin knows what happened to him . . .