system
Given the following post, respond as if you're commenting on Reddit:
human
[Highlander] What makes an immortal "powerful"?

I'm thinking more about the chronicles of Duncan McCleod, not his clansman Connor (whom I'm less familiar with). 

I've heard a lot about "when you take an immortal's head, you take his power". And I've also seen the videos of the "quickenings", when heads are taken, which are quite destructive (and more destructive for older/more "powerful" immortals).

I also recall seeing some documentaries where the dead immortal sort of takes over the live immortal (Duncan McCleod was once taken over by a "dark quickening", where he effectively became evil, and another where Richie Ryan took on characteristics of an immortal he killed, including falling in love with his girlfriend).

All that being said, I don't understand what they mean when they say, "when an immortal takes another's head, they take his power". It seems to me that Duncan McCleod is "powerful" because he trains like crazy and has trained with some of the greatest masters. Richie Ryan is able to kill many immortals that are much older and more supposedly "powerful" than him, likely because he trains with McCleod.

TL;DR: What does an immortal's "power" actually refer to?
gpt
It's the sum of an immortal's strength, knowledge, experience. Everything. Basically all the things that made them a person, their soul you might say. Since immortals, true to their name, live a very long time they tend to build a much greater reservoir of knowledge and experience than normal people, not to mention any they might have collected from killing other immortals.