Mage the Ascension is one of those games that can accumulate a lot of house rules, but I'll try to keep them to a minimum. Though M20 recently came out, we will mostly be using the Revised edition of Mage because…it's just better.
One important difference between M20 and Revised is that Revised uses a Resonance rule, where every Mage has at least one Resonance trait that colors their magic and gives it something of a fingerprint that others can read. The amount of Resonance you accumulate will play into whether you end up becoming a Marauder, and you will be able to use your Resonance as a quick and dirty path to power through hubris.
On any Arete roll, you can add in dice equal to your Resonance. This automatically imposes an extra point of Paradox, even on coincidental magic without witnesses. In essence, you are forcing your magic to work with more than just your will, but your will-to-power.
When Paradox backlashes, one of the things that can happen is that you can gain another point of permanent Resonance. Each point of Resonance has a problem associated with it, however, and the more Resonance you accumulate, the more problems you have. Each point of Resonance is the equivalent of a 1pt Flaw. So if you have 5 Resonance, on the one hand you have 5 bonus dice if you want to use them. On the other hand, you have accumulated an additional 5pts of Flaws. (You are also almost certainly a Marauder, attracting all sorts of unwanted attention) When you get a new point of Resonance, the ST has the final say on what flaw comes with it.
If your Resonance total is higher than your Arete, you are on your way to becoming a Marauder. When you use your Resonance in a magical effect, it also increases the 'fingerprint' you leave afterward for anyone with the right perceptions to see (Awareness, Kenning, the Prime sphere, etc.)
It is possible to buy off your Resonance, as you might with any Flaw, with in-game justification and 2xp per point of Resonance removed. It is much more difficult to remove that last point, but some say that is the first step toward true Ascension.
- Cross "Do" off your character sheet; it is a stupid, broken ability.
- You have health levels equal to 7, or 5 + your Stamina score, whichever is higher. Additional health levels are Bruised health levels with no wound penalty.
- No damage rolls and no soak rolls. If a weapon hits, it does its damage rating + extra successes in damage. You mark off health levels equal to that damage. So an attack is just an attack roll and a defense roll, followed by damage if the attack roll succeeds with at least 1 success. (Normally Mages cannot soak lethal damage anyway, so this is actually not too far from the RAW) One change this results in is that all melee attacks are dealt with Strength rather than Dexterity.
- You can maintain one magical effect that requires your concentration. If you try to maintain additional effects, each one increases the difficulty of all Arete and Mental rolls by 1. So, if you are trying to actively maintain 4 magical effects, your Arete and Mental ability rolls are at +3 difficulty.
- Just as you can reflexively dodge an incoming attack, you can reflexively defend against an incoming magical attack. You roll your Arete against a difficulty of 5. You can only do this once per round. This is a defense against direct attacks only – not against coincidental effects that might catch you, or against things like scrying on you, etc. Just defense against harm.
- I have a couple printed copies of the Merits and Flaws list, and I've crossed some of them out.
- I forget if this is in the RAW, but you get a specialty at 4 dots and at 5 dots in attributes, skills and Spheres. Two specialties from an ability never stack (but you might still roll 12 dice with two 5s and two specialties that both count).
- In keeping with the 1st Edition of Mage, Sanctum is not available as a Background.
- This is implied in the RAW, but you need to earn the ability to raise your Arete or your Spheres. You earn Arete by pushing yourself and experimenting with magic, and you earn new levels of Spheres with research or studying under a more knowledgeable teacher.
- Mages understand the world them in Mage terms, meaning there will be versions of Vampires and Changelings and so on that are more connected to Mages and whose abilities function as some form of awakened magic.