The Fisher King is an Arthurian legend of a king whose health affects his land. This can be used in a few ways for fantasy settings, including racial and creature design.

First is using the legend as is. A king or all kings are tied to their land and if they are ill, it is ill. Killing a king does not cause the land to wither and die, but rather the next in line takes on the supernatural tie. In some cases the ruler can affect the land and his people (Birthright, Distant Horizon's Eclipse) and in others it is just his health.

The second uses a god or other very powerful supernatural being. This obviously makes stability (for weal or woe) much longer but also means the PCs have less of a chance in changing conditions. In this case the tie might be so strong that all born within the borders of the affected lands are altered by the magic that comes from the being (so a blood aura effect over the entire nation or region). Templates and/or racial traits come into play here. If all lands are affected, this allows for settings where basic characters may be more powerful than normal without screwing with balance. If the gods are of different strengths, then the people may be unbalanced towards each other. Telling where someone is from is much easier in such worlds.

The third way replaces a person with a thing. It could be a sacred mountain, temple, statue, lake or sea, star, relic, artifact or even catacombs. With the last one, the number of dead may dictate the power of the nation. It may be only those who died peacefully, those taken in battle or the number of royal and noble bodies. As for relics, just watch the second Indiana Jones film.

A fourth way is to use the people. If they are happy and healthy, the land reflects this. Places where evil holds sway are dark by their very nature. The reason orcs are nomads is they can't help but destroy the land slowly, over a matter or years or a few decades.

One place I would like to see use fisher kings is Ravenloft. For the most part, the dark lords have little or no impact upon the land (other than cutting them off from neighbors). If the lord's personality or power seeps into those within his domain, it would make for a very different Domain of Dread.

Another is Spelljammer (at least for one sphere). Anyone who claims control over a body (planet, star, moon or asteroid) and can keep control will have the ability to really control its fate as long as he lives. Dragons, wizards and great warriors duking it out so they can have ultimate power.

So if you want people with a fire elemental template applied, a land controlled by an efreet or red dragon may be the way to go.