The Eternal Champion Saga is the informal name for the vast and sprawling Sword & Sorcery multiverse created by Michael Moorcock. These books were a huge influence on both Dungeons & Dragons and Warhammer Fantasy. The name derives from the use of "archetypical characters", the idea that different beings in different planes can be different incarnations of the same fundamental soul; the Eternal Champion is the archetype employed by the Cosmic Balance to ensure that the war between Law and Chaos can never end.
The Eternal Champion multiverse is technically Sword & Sorcery, but is closer to High Fantasy than Low Fantasy; magic is very real and comparatively abundant, whilst the cosmic forces of Law and Chaos battle it out, seeking to completely consume each world in the multiverse - because this would lead to ultimate annihilation, as complete Order would create a static realm of nothing and complete Chaos would result in a tumultuous yet equally static primordial soup, a third entity seeks to preserve the multiverse in balance. It allows worlds to tip one way or the other, but always stops them from being ultimately consumed. Despite this outright pursuit of their individual goals, Law, Chaos and Balance are seemingly non-sentient - or at least their minds are beyond the ability of humans to understand.
As part of the setting's "weird fantasy" basis, it's also a Science Fantasy metasetting, with individual worlds varying from fantasy worlds ruled by sorcery, to sci-fi worlds ruled by technology, to worlds where both coexist.
The Eternal Champion, as established above, is the archetype wielded as an agent by the Cosmic Balance. Whenever Law or Chaos is gaining too much strength, the Eternal Champion will be sent to defeat it and restore the balance. As such, this soul exists on every world and in every time period, each individual version doomed to a life in which they will be surrounded by strife and destruction... although, admittedly, they do often see peace for stretches - sometimes pretty long ones, at that. Their loneliness is somewhat abated by the fact that the Cosmic Balance has established two other archetypes to grant them aid and succor; the Eternal Companion, destined to aid and assist the Eternal Champion, and the Eternal Consort, destined to be their lover.
Of course, this series leaning somewhat towards the grimdark, an Eternal Champion doesn't necessarily know what their purpose is, which can bring them much suffering as Law and Chaos seemingly both turn on them for no reason. And then there are the poor dumb bastards who learn their destiny and try to fight it...
Regardless of incarnation, the Eternal Champion is always a mighty warrior, as well as being adept in strategy and tactics. They are usually adept leaders of fighters, if often unwilling to do so. The Eternal Champions often wield swords with black blades, but this is more an affectation; some Champions have wielded Black Swords that were powerful entities in their own right, such as Stormbringer.
Normally, the incarnations of the Champion do not interact and are treated as serial incarnations for facets of a single consciousness, but various cataclysmic events (such as the end of a Cycle of Cycles of the multiverse's progression, or an invasion by hostile entities from an entirely separate multiverse) may bring them together in various ways. This is very dangerous (it causes strains on the fabric of the multiverse as well as mental issues for the Champion), but amplifies the Champion's power enormously.
The most famous Eternal Champion is Elric.
A LOT of shit can be directly placed on the existence of this saga. Two of the most obvious? Well, alignment in Dungeons & Dragons was literally based on Moorcock's Law/Balance/Chaos trinity, before Advanced Dungeons & Dragons stratified it further with a Good/Evil axis. And then there's Warhammer Fantasy]... fun fact: you know what the symbols for Law and Chaos are in the Eternal Champion's multiverse? Law's symbol is an arrow pointing in a single direction. Chaos's symbol? Multiple arrows pointing in different directions, most elaborately in the form of a wheel with eight arrows coming out of it like spokes... sound familiar?