Very recently it was brought to my attention that this little gem from 1986 existed. I feel like history hasn't ever bothered to note it in spite of B/X's extreme popularity with OSR and non-OSR grogs alike. This game was Moldvay's attempt to pwn AD&D in 10 pages. And honestly while it's incomplete, and really requires AD&D understanding to use properly, it does what it sets out to do. It proves Moldvay knew what he was doing as a designer more than Gygax and Arneson (well, Arneson was never really a designer, more of an idea man). I've come to believe there have been two absolute game design geniuses working at TSR at one time, Tom Moldvay and Aaron Allston. Everyone else was merely treading water next to these giants. Challenges wasn't made for TSR, it's essentially Moldvay's unreleased "Companion" ruleset. A step toward a more advanced D&D. It's an intriguing design languishing in relative obscurity.
It's not really existent in a playable form, it's mostly like a section of coded notes. It's well worth checking out to get some visibility into one of the giants of D&D's thought processes.
In other news Kyle Brink was quietly fired from WotC after drawing the short straw to be the public's punching bag well over a year ago. The height of his career was making an openly racist comment on some random asshole's podcast as a part of the OGL misery tour he was forced to do. And with his firing, absolutely nothing of value was lost.
