There can only be one.
I was watching Highlander today, and thinking, "How would I implement one of those "highlander" immortals in Runequest?
The big problem is figuring out how high an immortal character could improve his skill level. The "highlanders" are even worse, since when one of them kills another, he gains all of his foe's experience.
I did some calculations, and concluded that a 400 year old "highlander" could very easily have a Sword ability at 720% or even higher, much much higher. With a 72% chance of getting a critical hit, I ought to make up rules for Ultra-Criticals.
Fortunately for Buffy the Vampire Slayer, Vampires are a lot easier to handle with my theories, since they don't acquire the experience of their defeated foes, and they spend so much time whining that they don't raise their skills as rapidly in any event. A 400 year old vampire would be more likely to have a combat ability at around 120%.
My theory is that a character's experience increases by the square root of his age, so if character A is four times as old as character B, she has twice as much experience. For a human-level intellect, advancement slows down to 1/3 after reaching 100% skill level, and I am judging that a motivated human can reach approximately 100% skill level over a 50 year career. (Most don't, but I can assume that vampires and highlanders are motivated.)
Vampires are only active for half the day, and spend a lot of time whining, so they only advance 1/3 as rapidly as a "normal" immortal. A 400 year old vampire has lived for 8 times as long as the benchmark 50 year career, so he's gained 2.828 times the experience. Subtracting 1 and then multiplying by 100 to represent the first "time" was used to gain 100%, he's got 182.8 more "points" which are divided by 3 because he's advancing only 1/3 as rapidly after reaching 100. That's about 60.9, but I divide by 3 again because vampires advance 1/3 as rapidly as other immortals, so he has 20 points over 100 or about 120%.
An individual may vary considerably from the average, of course.
A 400 year old highlander will, on the average, work his skill up to 160% by his own experience. But a highlander can also gain the experience of his defeated enemies. I assume that if a highlander defeats a more skilled highlander (which is probably not likely), he gains the higher skill, plus the benefit of where he's learned things his foe has not. If he defeats a less skilled highlander (which happens more often), he's already learned most of what his opponent learned, but there may be a few tricks and techniques that are new to him. So I'll assume a highlander will be able to add 1/5 the lower score to the higher score, and that will be his new skill level. Thus if a highlander with 160% defeats another highlander with 160%, he will gain 32 points, and be at 192% after the battle.
I just guessed that the average highlander will have to defeat 2d6-2 highlanders every 100 years, or an average of 5. Good highlanders probably don't get into as many fights, so maybe they average 1d6-1 victories, while evil highlanders average 3d6-3. Just arbitrarily, I assume the average highlander gains 20 points after a battle. So a 400 year old evil highlander will have defeated 28 enemies and gained 20 points from each, for a total of 560 points, plus the 160 points he earned from his own experience. His total sword ability is something like 720%.
Has anybody else considered how to determine the abilities of ancient characters? Do my numbers seem reasonable or way off base? I'm open to any feedback or considerations.