A core component of K20 is that the PCs all know the math. This goes for the monsters as well. One of the outcomes of that decision is that anything with an intelligence score of 3 or higher can be playable. This leads us to a few design choices. If the players can be a goblin, and players don’t have to be evil, are all goblins evil? As a result, K20 downplays alignment quite a bit. it crops up now and again, but there are not mono-species empires, and every goblin isn’t evil.
The second major realization this leads us to, is that if the players can play a goblin, and they goblin and frost giant both use the same system, why can’t the players be a frost giant? The outcome? K20 has the traditional 3.5 core races, reworked as origins, but also rules to play essentially anything in the monster manual, if you really want to. Golems, skeletons, and other insentient beings may require some homebrewing on the DMs part, but otherwise, go for it.
From the above, we realized monsters need classes. We could build every monster as a Barbarian or a Wizard or what have you, but that’s a lot of work and doesn’t really meet the monster needs. Instead, we use monster classes. These are simple to use, 20 levels long, and give monsters abilities they need to do their jobs.
Lastly, K20 has moved on from the inaccurate, confusing, and uncomfortable language of race in RPGs, and uses Origin for the type of frog person you are playing. There’s a lot of debate around it, but ultimately retaining the language around race doesn’t add anything, so why keep it?
An Essay on Monsters as Origins
In 3.0, 3.5 and prior editions wanting to play a monster was always looked down on by the designers of the game, they would usually say something about how you are doing it wrong. Their solution was to make the system confusing, opaque, and under-powered so people would not want to. The Problem? People still picked it, despite being underpowered, and were then unhappy. 4e and 5e approached this problem by diluting the actual thing folks wanted to play, and presenting a PC compatible version. That is somehow worse, and we will simply ignore it using the 3e approach.
Level Adjustment, ECL, CR, and Racial Hit Die. This is traditionally one of the most confusing aspects of 3.5, and most players do not really know the difference between them or how they relate. Most of the time this is not important, but every so often you end up with a Were-Tiger Sorcerer 4 in a party with level 8 PCs complaining about being underpowered, and at the same time, being told by the other players that they are actually getting away with something and should be thankful.
Luckily for us, these confusing, sort of interchangeable, sort of related things are also terrible mechanically. A Lantern Archon is CR 2 and has 1 hit die. That means I can play a lantern archon sorcerer as a level 1 PC. That means I get a perfect fly speed a couple of CL 3 SLAs, DR 10, some immunities, and the ability to teleport. A kobold is CR ¼ and has 1 racial hit die. I can play a kobold sorcerer as a level 1 PC and get +2 dex, -2 con, +1 natural armor and light sensitivity. One of these things is not like the other… If I want to fight them, the CR 2 Archon is not a hard fight for my first level party. They blast it with a magic missile and end it. For the same CR, I can fight 8 Kobolds who bombard me with slings, and we have a real fight.
For K20 we are simplifying everything. Any entry in the monster manual (the GM still needs to approve all characters as normal, looking at you celestial gelatinous cube monk) is eligible for play as a character.
To make a monstrous PC, subtract the monsters CR from the level of the PC you would like to make. If the result is less than 0, you cannot play this character. If the result is 0, you can play one, but do not get to choose a class until next level (you get racial classes as normal). If the result is at least 1, this becomes your ECL. You can enter a class, as normal, but when your total ECL is enough to put you at the next tier, you automatically skip any remaining levels in your class and go to the next tier as normal. You can never gain those levels. This means if you are a Lantern Archon Sorcerer from the 3.5 monster manual (CR2) in a 4th level game, you have 2 class levels. If your origin only gives 1 or less racial hit die, they are dropped as normal.