Essentially, the rules are just RuneQuest magic added to Pendragon. (When I designed the rules, Pendragon didn't have a magic system. And its Celtic magic doesn't seem appropriate for Glorantha.) This allows using standard supplements like Gods of Glorantha.
The most complete and recent version of the rules is a 28-page insert in Enclosure 1. I described a slightly earlier version of the rules in issue 6 of the fanzine Tales of the Reaching Moon.
Skills are ranked 0-20+. To use a skill, roll d20. If the number is less than your skill, you've succeeded. If the number is exactly your skill, you criticalled -- an excellent success.Sometimes skills are opposed. Both characters roll. The higher success wins. Criticals are considered to be 20.
Example: Suboti attempts to sneak past a guard. She rolls a 2, under her Stealth of 10. Unfortunately, the guard rolls a 4, which is both under his Awareness of 8 and higher than Suboti's roll. The guard spots her.Combat is an opposed roll. Whoever wins gets to roll damage (usually 4d6 or 5d6). If the loser happened to make his roll, he gets to block part of the damage with his weapon (3 points) or shield (9 points). [Note: in standard Pendragon only shields can parry, for 6 points.] Any damage that gets past the parry is reduced by armor, and causes a wound. If the resulting damage is CON or greater, it's a major wound, which effectively takes the victim out of the fight, and usually has a long-term effect. If the amount of damage (before any reductions for armor) is over SIZ, the victim must make a DEX (or Riding, if mounted) roll or else fall down.
Wounds take a long time to heal. A wound can only be bandaged (First Aid) once, and healed by magic once. This will handle minor wounds, but severe wounds can take weeks to heal, and only do so under the tender ministrations of a Chirurgeon.
A character's personality can be measured by traits (like Honest or Generous) and passions (like Hate Trolls). Behaving in accordance with a trait earns a check. Sometimes, traits are rolled (for example, one character's Lustful might be opposed against another's Chaste).
The GM assigns skill or trait checks. Over winter, players roll for their character's checks. Rolling over the skill or trait means it increases by one. Training is also done during winter (in fact, it goes on year-round, but this abstraction works because there's usually only one or two adventures per year).
Another rules variant for Glorantha is Nick Eden's Moon Wars.