Characters have attributes in the D&D range: typically between 3 and 18 or maybe 1 and 20, with the average around 10 or 11. They might have special abilities or magic. Most spells can be used roughly once per adventure.
Task resolution is d20 plus attribute. If the character does not know what they are doing, we might use half attribute or +0 instead. The d20 explodes upwards (on a roll of 20) and downwards (on a roll of 1). Each task will have a particular difficulty, 20 for many adventurous tasks, to succeed at it.
A starting character has d6 hit points. A successful attack (as established by the task resolution rules) takes away d6 hit points. If the character does not have enough hit points to absorb the attack, they might be wounded.
The actual game is, of course, not these resolution rules, and not in the character advancement rules either, but rather in how these are used. I am not explaining that here.
Using a character
These rules are made specifically for one-shots and for players who bring their own characters, of any rules system with roughly the correct power level. If necessary, you can also make a new character.
When using a character, if their attributes are not at the correct scale, convert them to the D&D scale, so that average score is about 9-12, low score is about 1-5 and human optimum about 15-25.
The set of attributes you have is okay. If something is missing, we can always roll it in play with 3d6.
For the special abilities and spells, we’ll take them into account fiction first: you describe what they do in the fiction, we figure out how we take that into account, same as anything else in the fiction. Note that by default your spells are usable once per adventure; in case the adventure spans large amounts of time there are alternatives to get them back faster, and you can take risks to cast them again, but we’ll get to those as necessary.
We also need hit points and level for the character. If the character already has hit points or level, use those. If not, they are first level and have d6 hit points. You can roll now or when you take damage for the first time. Note that hit points are a floating concept; they recover by resting and are lost by stress and to avoid injury and other problems. There is no maximum hit points.
Advancing a character
You gather experience based on accomplishing adventurous goals you set up for yourself. The default is getting rich, one experience per gold piece with second level at about 2000 experience, but other alternatives are obviously possible and encouraged.
At the end of a session you get to try to improve one of your attributes, or perhaps learn or master a skill. If you have more level than special abilities, you might learn a new special ability.
More elaborate rules
The idea behind the system is that we build on, or maybe take away, the rules as we play longer and get used to the mechanical framework and philosophy of play. We are unlikely to see too much of this in a short game. The mechanical choices we have made are just that, while the fundament is the philosophy of play.
This is heavily based on a house system of Eero Tuovinen.