13th Age is from Rob Heinsoo and Johnathan Tweet, and is the combination of 3rd and 4th edition you might expect, but with some pretty major twists.
1. There are 10 levels, not 20. You ramp up, though - when you roll weapon damage, you roll a number of dice equal to your level, and might roll MORE dice depending on your class. HP ramps up as well. 2. There's no XP, you just go up a level every 13 encounters or so (this is actually the same as it should be based on 3rd and 4th encounter math). 3. Background instead of skills. Players are encouraged to make these more specific than just "Butcher" or "Wizard." After all, if you're "Apprentice to Mal-vel, Butcher to the Archmage," then you can use that background not just for non-combat use of a cleaver, but for things related to wherever you decide Mal-vel, or issues related to the Archmage. 4. One Unique Thing. Every PC gets one of these things and, whatever it is, it's always true. This could be "Last Son Of The House of Antresuga," with the campaign at least partly focused around that. I had a monk character who was Definitely Not A Bear. Yes, he was very large and hairy, liked eating honeycomb to get at the grubs and his melee attacks did slashing damage, but he was Definitely Not A Bear. It was just ... fun, a way of making him stand out from the crowd. 5. "Flat" Monster Design. The DM chooses one attack from the two, maybe three, that the monster has available, rolls a d20 and then does a flat amount of damage, plus possible extra effects. And that's IT. No long lists of spells or giant list of abilities, just a few, limited options. Which means, honestly, that the options are that much greater. You can have an ancient red dragon in a fight with three or four guardians, ALL of them with a unique set of abilities, and you don't get lost because while they're unique, they're limited to just those abilities. 6. The number on the d20 matters. This might be my favourite part - monsters will have notes like, "Natural even hit - 6 ongoing fire damage," or "Natural odd miss - slashing damage equal to monster level," and that's a reference not to the total of the roll, but just what shows up on the die face. Heroes have these as well and classes like the bard and fighter get LOTS of them, so just about every time the player rolls, they'll have options to choose from for what that means.7. The escalation die.
Fights should start with the players missing MOST of the time. Like, 60% of the time, but on the second round you put out a d6 turned to a 1 - everyone gets that bonus to their d20 rolls. The next round, it goes up to 2, then 3, until players get a +6. The net effect is that players learn to not use their best attacks on the first turn, when they're likely to whiff, and let's them take big, stupid risks.
8. It assumes familiarity with f20.
"F20" is the general term for games where you explore exotic locations where you have exotic encounters, with rolls of the d20 as the primary driver of success. 13th Age is by two professional game designers, and while they do explain what they're talking about in some frankly hilarious sidebars, it assumes that you're looking to play this game for is core activity. This is a disadvantage for people who aren't terribly familiar with games of this type, but a serious advantage for those who are. The system's not exactly stripped down, but it's laser-focused on its core activity, so you can quite easily bolt other subsystems onto it without impacting the math too much.
https://www.13thagesrd.com/
