Artwork by Jason Bulmahn
I love Pathfinder. I love its versatile options for characters. I love the plethora of material, both official and third party. I love the myriad adventures and adventure paths, and all the cool settings. I love the cool hybrid classes like the alchemist and the magus. I love the Bestiaries and the Tome of Horrors for the 1,000+ monsters these books present as neat options, both classic and original.
But there are things I don't like about Pathfinder. Namely how the rules can stack up over time to the point that they can bog down the game. This is especially prominent in combat, where managing large numbers of monsters can be tough, and even low-CR opponents can get a lot of hit points. The archetype of a master swordsman cutting down hordes of enemies in mere seconds is very hard to do in Pathfinder, unless you're a spellcaster with an Area of Effect spell. Stat blocks can be a pain to build, especially when it comes to spellcasters.
Rule Zero: Underlings is an attempted fix to the above problems, by designing a new kind of enemy known as an Underling whose primary purpose is to be quickly-statted mooks who challenge the PCs in groups of 4 or more and yet can be easily defeated while still outputting a reasonable amount of damage. As one who's been using Rule Zero's content for several months in my Pathfinder games, the Underling option works very well for its intended purpose.
Basically Underlings are streamlined NPCs whose core abilities are factored off of the Group Challenge Rating, which represents 4 Underlings of the same type. Regardless of whether the Underling's are meant to represent giants, mages, vampires, or the like, Armor Class, skill bonus, attack bonus and damage, saving throws and Wound/Kill Threshold are based off of the Group CR. Instead of tracking hit points, Underlings are dropped if a single damaging attack exceeds their Kill Threshold, or get wounded twice (like Kill Threshold, only half value of the Kill Threshold); damage below this does not affect the Underling. This, combined with a static attack and damage dice (the latter which can take the average in lieu of rolling) really speeds up the creation process, it's far quicker to design an Underling stat block than a typical NPC, and their low power levels make fights with a large number of opponents more manageable if they're used as the excess minions.
An Underling's "race" (which can represent humans and dwarves as well as broad and popular monster types such as giants and demons) factors in special abilities, which skills their "Class Skill Bonus" applies to, and which saving throws have the "good" and "bad" progressions along with some other minor things. Finally, Templates are things which can grant Underlings a special attack or mimic the class features of iconic archetypes such as Sneak Attack. Generating spellcasting Underlings is a simple affair of choosing a total number of spell levels from the cleric or sorcerer/wizard list equal to half the Group CR, and said spells are cast 1/day as spell-like abilities (or at-will for 0 level spells).
One of the best features is that there are Underling races for otherwise high hp, high CR monsters such as the Fire Giant or Mummy. As the Underling Group CR is a recommended guideline rather than a restriction, there's nothing preventing you from making CR 12 kobolds or CR 1 Ogres. With this, even a Fighter of middling ability can cleave through several towering brutes in a single round, something which doesn't really happen in core Pathfinder unless you're building a min-maxed Barbarian ubercharger or something to that effect.
Although it's a short 10 pages, Rule Zero: Underlings is more than worth its $3 price tag, and it's one of my favorite DMing tools for the game. The toolbox of races and templates are versatile enough to mimic all kinds of fiendish minions and monsters, and even evil assassin-thief archetypes meant to challenge mid-level adventurers can be built in 1-2 minutes with these rules!