It's based around the BRP system written by Chaosium for the original RuneQuest, which was then adapted for Elric, Call of Cthulhu and others. Essentially, you have seven abilities which more or less map to the six D&D abilities (Strength, Dexterity, Constitution, Size, Intelligence, Power and Charisma, where Power is basically Wisdom), with the same typical ranges for a human.
Almost everything else is either a skill with a percentile rating, and each skill has a starting value equal to two of your abilities added together. For example, your ability to use a sword and shield in combat would be a skill called "Sword and Shield", which would default to STR+DEX. You either roll or use point buy (80 points, spend on a 1:1 basis, nothing lower than 6 or higher than 18 for a human) to determine your stats, pick a culture and a profession, which gives you bonuses to a bunch of common skills and access to a bunch of advanced skills, and then you spend points on skills on a 1:1 basis (maximum skill rating of 70%), and you're done.
Edit: Also, forgot to mention, difficulty is determined by a bonus or penalty to the skill for the purposes of the skill check. A difficult roll would be -20 to your skill, while a simple roll would be +20. Taking longer to perform a task makes it easier, which gives you bonuses.
Magic uses magic points, and if you run out you risk falling unconscious. There are three types of magic: Common Magic is, in the default setting, something that just about everybody knows a little bit of, but I'm tempted to rule that it's an advanced skill and only available to people who actively study magic. It has some basic buffs and debuffs, some basic attacks and healing, and is generally a good starting point for anybody who wants to use magic in general.
Divine Magic is semi-Vancian, and access to higher levels of magic requires that you improve your standing with your religion. Lay members are just worshippers, and get no divine spells. Initiates are trainee priests, and receive basic divine spells, Acolytes are advanced trainees who receive more advanced divine spells, Priests receive more advanced magic still, and high priests are really powerful. You have two skills governing your use of magic:
Pact (God) determines how powerful your spells are, and the higher it is, the more like your god you start to act. You roll against this to regain spells. Early on, you can only do this at a temple or shrine, but as you become more powerful, you can do it anywhere you like. In order to get Pact (God), you must devote a certain amount of POW to that God. This reduces how many MP you have for other things, but doesn't affect your skills. The higher your rank within your church, the more POW needs to be devoted.
Lore (God) determines what you know about your religion, and is what you roll against to actually cast a spell. If you succeed, you lose that spell until you regain it as above. If you fail, you don't lose it. On a critical success you cast the spell and keep it for later use; on a fumble you fail but lose the spell anyway.
Sorcery is the most flexible, and as a result potentially the most powerful form of magic. There are two skills for this kind of magic: Manipulation and Grimoire. Manipulation is never rolled against, and instead just determines how much you can modify your magic by, while Grimoire is a separate skill for each individual spell book you have access to. You roll against Grimoire to cast a spell, and it costs a number of MP to cast based on how much you modify the spell by.
In combat, you get a certain number of actions per round based on DEX and INT, and your initiative is based on the same. Having a weapon or shield in your off hand gives you an extra action. Weapons have a minimum STR and DEX to use effectively. Your HP is locational, and you're highly unlikely to go into double digits in it, making combat somewhat more dangerous in RuneQuest (for this reason, I'd be cutting out several of the encounters from the conversion; particularly where you'd be fighting multiple huge monsters at once). Finally, active defence (parrying a blow with your sword, for example) requires an action.
Experience comes in the form of Improvement Rolls, or IRs. You roll a d% and add it to your INT; if you get above your current skill, you add 1d4+1 to the skill; less and you just add 1. Your skills can reach over 100; when they do, you start rolling d% added to a fraction of your INT and trying to get over 100. This means that the better you are at a skill, the longer it takes to noticeably improve further. High CHA results in extra IRs, as you get other people helping you to improve, and low CHA results in fewer IRs because nobody wants to help you. They can also be spent on abilities, but it costs multiple IRs to improve an ability.
POW is the basis of the Persistence skill (basically a Will roll); CON is the basis of the Resilience skill (Fort) and DEX for Evade (Ref). Because the game isn't class based, these are limited only by how much you choose to improve them.
Finally, Hero Points allow you to do cool stuff. You start with 2, and you earn more as the game progresses. You may use one for an extra combat action (let's say you're on the receiving end of a particularly vicious attack and you're out of combat actions, you could pay a hero point to be able to defend yourself). You may use one to avoid being killed and/or maimed as the result of an attack, you may use one to receive a clue from the GM when you're really stuck, and you may spend them to learn a heroic ability.
These abilities cost 1 MP to use, even though they aren't actually magical in nature, and include things like Arrow Cutting, which lets you parry ranged attacks with a weapon, Disease Immunity, which is exactly what it says on the tin, Reincarnation, which allows you to be reborn as a different creature one time only and Wall Leaping, which allows Prince of Persia style acrobatics. They all have ability and skill requires, and must be learned from teacher (I'd provide opportunities for this as you start gaining enough Hero Points to take advantage of them).
