Companion Set

The Companion Set, or Companion Rules, are the high-level rules in the Tom Moldvay spinoff of the original Not-Advanced D&D rules. They kick in around 15th level after the Expert rules taper off. Frank Mentzer, after a thorough retcon of Basic and Expert, put this out around 1983ish.

Since the targeted levels overlap high-level AD&D, there is where - finally - the party will meet the Ghost and the Beholder. (Tho' not yet the Lich. Nor any demons.) Also here are some high-ish difficulty monsters mooted here and there in the Expert line, like X5's malfera. We do have to hit the editors for klutzing up the alphabetic order of the monsters however, including - laughably - the gremlin.

Here come the seventh and eighth level tiers of spellcasting, where mages really start to kick some ass. Also brought here are the Druid, and some way for the Elf and other demihumans to keep progressing along with the humans - by way of alphabet-levels, in lieu of numeric-levels. Well not every idea here is perfect.

At 15th level on up, really 9th on up, the PCs should be considering doing a Conan and ruling domains of their own, delegating other - lowlevel - adventurers to do shit like goblin hunting. There are some brilliant ideas in the Companion DM section on what PCs can do in the meantime; most salient, that domain-ruling mechanic (better keep that Confidence Level high, folks!) and also the WAAAGH Machine, sort of a "Take Two" for old Chainmail. Although the 'Machine was somewhat Strategic for tactical tabletop gamers' tastes, so BATTLESYSTEM would serve that latter niche.

Beyond 24th level, PCs would be moving on from Mature to Epic, so the Master's rules kick in after that.

The modules in this system began CM-. Some of them are good, although it all starts out as a bare showcase for the system.

Overall, the Companion Rules were awesome, or at least formed a serious competitor for high-level AD&D. They both scaled better than AD&D 1e and provided some large-scale heroics for parties beyond just bashing skulls.