A comic book I was pointed to posited a monster that gained power from the ego of a particular protagonist. The more self-assured and self-important the hero allowed himself to get, the stronger the monster became. And since this particular hero had an enormous ego that he insisted on having everyone constantly stroke and feed, the monster became very powerful indeed. The only way to make the monster beatable was to teach the protagonist some humility.
Now creatures that steal and absorb POW are nothing new to BRP fantasy games. Ego -- the sense of one's own importance -- is another matter. Especially if your self-confidence is largely unjustified (you believe you are better, smarter or more powerful than you really are). POW is something entirely different than that -- humble, self-sacrificing characters frequently have very high POW scores, and the POW of the egomaniac warrior-nobleman could well be average or worse.
The question is how to make an intangible roleplaying element (the ego of the player-character who believes himself to be more important than he really is) into something tangible (the monster buffing itself as the hero "flexes his muscles").