Rollmaster, as it would be more accurately described, is an ancient classic from the Dawn Times, brought to you by those anal-retentive lunatics at Iron Crown Enterprises. If you picked up this game in 1980 and started making a character immediately, then you should be almost ready to play this Friday.
Play involves describing your action, then rolling a d100 and waiting around for a few hours for the GM to read through many different branching charts to figure out which very specific instance of bodily harm was inflicted on the enemy you may or may not have hit depending on which charts were being cross referenced against one another. For example, the "dagger vs. breastplate" chart is very different from the "dagger vs. loin cloth" chart, and the degree to which the dagger disembowels the person in the loincloth versus removing the nose of the breastplate wearer is similarly affected by the letter column of the critical hit (most hits are critical hits because fuck hit points) and whether the dagger has various properties (i.e. a regular dagger may do puncturing, slashing, or bludgeoning depending on how it interacts with the target's armor, while a holy dagger does damage on the holy chart, unless against undead, in which case it rolls on the slaying chart, and a cold dagger on the elemental damage chart, except against fire elementals, etc.), and any other armor the person may be wearing (i.e. helmets, codpieces, mouth guards, etc.)
Some of this could be mitigated by xeroxing the relevant charts. What couldn’t be was that character death is about as omnipresent as it was in AD&D, as a series of bad rolls (generally anything over a 100 on the to hit roll, coupled with a roll of exactly 66 or over 85 on the crit chart) will lead to instant death, which is terribly frustrating because (as mentioned above) character generation doesn't take 30 seconds like it did in AD&D, leading to many characters having gigantic extended families of people all with the same class, gear, personality, etc.
Can be a fascinating and hilarious study in brutality and body horror if you've got a GM who's been playing for 20 years and knows the charts like the back of his hand. Nigh impenetrable for new GMs.