Archnemesis.

It's almost a redundant word. No, scratch that: it is redundant.

arch- - the etymology of this runs back through Middle and Old English, back to the Latin archi- and through that in turn to Ancient Greek where it can be seen as archo and I won't even try to get the character set right there, or to pretend I have actual command of that reference from a base of knowledge. But back then, I'm told by various sources, it mean 'to begin, to govern, to lead' - implying either a start or a singular lead position. The English use, lo these many years later, seems to indicate "chief" or "most extreme" or "highest/most important." That's the use we find in archnemesis.

The funny part is that nemesis itself has definitions which include "archenemy."

Hence, redundant.

It also means 'instrument of downfall,' implying a linking of fate.

Why is it thus? Well, one of the largest if not the most definitive uses of this word in modern times is in the realm of entertainment fiction - comic books and films and television, and all that implies. It's used most often in superhero tales.

The archnemesis - literally, the arch-arch-enemy, in this case - is in these uses the pivotal antagonist of the main character, who is usually a singular and special person. It's almost unheard of for non-supers or non-critical characters to have an archnemesis. The implication is that only important people, who can make a difference, have them. Maybe this is so that if they fail to make a difference, or are prevented from doing so, we can ensure it's not their fault? Who knows.

In any case, the comic book world is littered with them, in famous matchups that, these days, even non-comics-nerds would recognize. Although the implication of the word is that the nemesis is the evil one, that surely depends on your point of view and who you ask, right?

Batman has The Joker, and this may be one of the earliest and most famous matchups. Superman has Lex Luthor. Spider-Man fights Green Goblin. Or, of course, you could say that The Joker has Batman, right?

But so forth and so on. Cappy has Red Skull. The Fantastic Four collectively have Dr. Doom as an archnemesis. Professor Xavier has Magneto. For a great take on sibling rivalry, Thor has Loki. Daredevil has The Kingpin. In each case, when a Mysterious Threat(tm) emerges for our hero, the first thing the hero will assume, or (if they're more measured) seek to eliminate is the possibility that the threat originates with their archnemesis.