Or, technically, I guess this is "The Six-Monster Bestiary and The Five-Element Treasury and The Rock-Cut Bazaar", to use it's full name. This new release by Jojiro is, in concept, an excellent resource document for an OSR GM - with both a bestiary, treasure generator, and one-page dungeon, it could turn any minimalist GLOG hack or other microgame into a full system.
But how well does it do in practice?
Disclaimer: this is based off of a reading of the supplement, not use in play.
The six monsters were chosen to be good introductions to wider concepts in D&D - a basilisk for save-or-dies, a carcass scavenger for paralysis, a demon spider for surprise, a gargoyle for immunity, a skeleton for undead mechanics, and a bear for just being very threatening.
Along with that, it has some adaptation notes: turning the gargoyle into a werewolf by removing flight, or a harpy by removing its immunities.
While I do think these are all important niches, I would say it misses a couple: a wizard of some kind to introduce monsters' access to magic, a hobgoblin or similar to show that monsters can be organized and tactical, and an elemental or other creature to introduce weaknesses (while the demon spider is afraid of fire, this isn't a core part of it). There are also going to be other niches both the creator and I have missed.
The monsters themselves are very well-presented and made to be easily converted between systems: they have both ascending and descending AC, and a guide is presented to convert their default unified Save score into either the classic 5 or the 3e-era Fort/Ref/Will set. They vary between having d6s and d8s for HD, which might cause a somewhat rough transition into some systems.
All of them use Onion Statblocks as described in this post, which is meant to make running the monster easy for beginning GMs.
The Five-Element Treasury divides loot into (unsurprisingly) 5 categories: copper pieces, gold pieces, Relics (incredibly profitable items that are heavy, fragile, or otherwise take effort to extract from the dungeon), Equipment, and Expendables (potions, scrolls, and the like).
For the latter three, it has d12 tables (except the Equipment table, which has 2 extra entries). The Relic table is less interesting than I would have hoped, focusing almost entirely on fragile items instead of heavy, sentimental, spiritual, or otherwise difficult items.
The Equipment table is all magic items except for two (padded shoes and thieves' garb, both improving thief skills). These are all somewhat basic, but usually have something better than just being +X weapons (the +1 Mace of Disruption dealing double damage to undead or the +3 Arrow of Slaying instantly killing a certain creature, for instance).
The Expendable table is disappointing, with only one of its 12 items actually being described. Six potions are presented (Clairaudience, Gaseous Form, etc.), and their durations are listed, but no mechanics are given for them. While this isn't a problem for experienced OSR GMs who have these all committed to memory, it's a shame given how accessible the rest of the book has been to beginners.
After that is the single described item, the Chime of Opening (which opens nearby doors when struck - all nearby doors), then five scrolls. The first is Cursed, which the book simply says contains "a curse of the referee's making", which I think is another piece of the treasury that could use expansion (at least one example curse, preferably a d4 or d6 table). The last four slots are spells containing random arcane and divine spells.
Along with the treasure tables, it has three categories of loot, each assigned to a type of monster - Intelligent Hoarders keep only the best, so their hoards have good chances of having gold, Relics, and similar, Shiny Hoarders collect everything they can, so their hoards are clogged up with thousands of hard-to-move copper pieces, though they'll still have worthwhile stuff buried beneath, and Incidental Hoarders only have what they have by accident, leaving them with nearly worthless hoards.
While the concept of the Five-Element Treasury is good, the Relic and Consumable tables need a second pass for more interesting entries (the former), or to be expanded with needed information (the latter).
The Rock-Cut Bazaar
The disclaimer again, because it's more important this time: this is based off of a reading of the supplement, not use in play.
The Rock-Cut Bazaar is a one-page dungeon meant to use both the bestiary and treasury in a true gameplay environment. It succeeds in this, but only somewhat - the treasury's tables are used once, but the bestiary is never referenced, and the dungeon's denizens are not given stats. While this could be seen as an example to a new GM, as they need to make use of the bestiary, I think it could have been made a better example by statting the monsters. Then, you would be able to show how far you can go with reskins and slight ability changes.
As a dungeon, the Rock-Cut Bazaar fits a checklist: a trap, a particularly dangerous monster (a bear ghoul), light faction play, and a future effect (if the players manage to cage a fool's gold-producing cockatrice). However, the map is very simple - a single central room surrounded by one-room branches, with no loops or secret paths.
The Rock-Cut Bazaar is fine, but given how little it takes advantage of the document it's a companion to, I see no reason to use it over other one-page dungeons.
While the Six-Monster Bestiary has an excellent concept, and could be invaluable to new OSR GMs or anyone moving to a super-light system, it has some holes that would be good to fix - unfilled niches in the Bestiary, simplistic relics and missing item descriptions in the Treasury, and the lack of connection between the Bazaar and the rest of the document.