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LexStarwalker : would you care to explain the effort and xp-spending rules, for us non-Numenera players?
And let me get this straight: if a player reduces a tasks's difficulty to zero, then it becomes an auto-success? And there are no die roll modifiers, just difficulty modifiers? (Wait, how do you hit difficulty 7+?)
Subtle differences. But interesting. I'm not sure they're practically different from the d20 rules, but I can see how they would create a different feel.
Last question: how do difficulty levels work? It's on a 1-10 scale, but who decides what's a 1 or a 10?
I will do my best!
Difficulties in Numenera range from 0-10. Your target number on the d20 is 3x the difficulty. You don't have to roll for difficulty 0. Difficulty 1 you need a 3 or higher, difficulty 2 a 6 or higher, and so on.
You can use Effort to reduce the difficulty of any action. You use Effort by spending points from a pool. If it's a Might action, you spend Might points, if it's an Intellect action you spend Intellect points, and if it's a Speed action, you spend Speed points. The first level of Effort costs 3 points, the second and third levels of Effort cost 2 points each. So 2 levels of Effort costs 5 points, and 3 levels of Effort costs 7 points. At first tier you can only use 1 level of Effort. Any Edge you have in that stat subtracts from the Effort cost. Each level of Effort reduces the difficulty by one.
Being trained in a task reduces the difficulty by one. Being specialized reduces it by 2. You can also have assets (similar to situational modifiers in d20). Each asset reduces difficulty by one.
So, say you want to climb a mountain, and the GM says the difficulty is 3 (target of 9 or higher on the d20). You are trained in climbing and you have climbing gear. Being trained in climbing reduces the difficulty by one, taking it to difficulty 2 (you'd need a 6). The climbing gear is an asset that also reduced the difficulty by one, taking it to 1 (you'd need a 3). Chances are pretty good you'd roll a 3 or higher, but you could always roll a 2, which would fail, and a roll of a 1 is always a mandatory GM Intrusion (although that doesn't necessarily always mean you fail, it's up to the GM). If climbing the mountain is really important, and you don't even want that 10% chance of rolling a 1 or 2 and failing, you could spend one level of Effort, which would lower the difficulty to 0. Then it would be an auto-success and you wouldn't have to roll.
In Numenera you can spend 1 xp to reroll ANY roll in the game. You can do this as many times as you wish, whenever you wish (as long as you have xp). This means you can spend 1 xp to reroll a failed roll you made, or a roll that any other player made. (or another player could spend 1 xp to have you reroll a roll you just failed).
There are die roll modifiers, but they're relatively rare and limited to +1 or +2. If you had bonuses adding up to +3 somehow, you'd reduce the difficulty by 1 instead (although this seems mathematically the same, reducing the difficulty could result in a difficulty 0 task which would mean you wouldn't have to roll).
Difficulties 7-10 are impossible without skills, assets, and/or effort. This is because, as you pointed out, the target number would be 21 or higher, which is impossible to roll on a d20. Numenera does not have an "auto-succeed" if you roll a 20 (instead you get a major effect). However, higher tier characters can make these harder rolls because they will be trained (-1 difficulty) or specialized (-2 difficulty) in the task, and/or have various assets (-1 difficulty each), and/or spend up to 3 levels of effort (up to -3 difficulty).
XP can also be spent on other things like getting a home or base, getting a contact, getting a source of income, getting a limited skill (either limited by time or by place, e.g. picking the locks in a specific structure) and other things. This is all in addition to spending xp for advancement.
The difficulties of 1-10 are all pretty well defined. Each also has a one word adjective associated with it, so once your players are savvy you can use that word instead of the number for better immersion. For instance, difficulty 0 (Routine) anyone can do basically all the time. Difficulty 1 (Simple) most people can do most the time. Difficulty 2 (Standard) is a typical task requiring focus, but most people can usually do it. Difficulty 3 (Demanding) requires full attention, most people have a 50/50 chance to succceed. Difficulty 4 (Difficult) trained people have a 50/50 chance to succeed, and so on. Most tasks should be no higher than difficulty 3 or 4 at the maximum. The higher difficulties are reserved for very dramatic scenes and encounters (like creatures you encounter, super weird tech you try to figure out, etc.).
It is true that from a mathematic perspective adding modifiers or reducing your target are the same, but the actual gameplay of the Cypher System and d20 are very, very different. I think most of us really need to play the game to really grok how fundamental and far-reaching that difference is.
There is definitely strategy and resource management involved, but if you like very mechanics-heavy games, this may not be the system for you. However if you're more focused on storytelling and want a system that's easy to learn, easy to adjudicate on the fly, and easy to teach, then you may love this system.