This one is a campaign setting, a magic system, and a story idea all in one.
Soul Taps. In a world of fantasy, let's call it Nadir, there are many, many races. Elves, dwarves, and humans are the predominant mortal races, but they are far from the only beings in reality. The Fae, both light and dark, exist in the shadowy, untamed realms of the world, and dragons and demons, angels and demons, all of them walk the world. Or rather, once they did in far greater numbers than they do now.
Mortals were all born with magical conduits throughout their bodies, conduits much like another nervous system within them. These conduits start weak in all magi, but through practice and dedication they could be expanded through training to allow the mortal to channel greater amounts of magic. Yet all magi had limits to how much they could expand these conduits, one that was inborn. A mortal might reach the limits of their power after only a few years of training, or they might find it only after decades. These limits frustrated many aspiring magi, for they never new how far their talent might extend. So many magi continually searched for ways to increase their power. The search for more power is what led to the first soul tap.
The first soul tap was an attempt by a mage to store power from a ley line within a gemstone, in the hopes that he could draw upon that energy. However, when he examined the result closely, he was astonished to find that he had created a miniature world within the gemstone, as the energy from Nadir formed into a familiar form. It was tiny, only a few hundred feet across the tiny world, but its time flowed more quickly than that of the outside world. The mage was distraught, however, when the tiny world collapsed only a few weeks later. He obsessed over it, and at last he decided that the reason the world was collapsing was the lack of a living mind within the world, maintaining its form. So he sought out a weak faerie and trapped the creature within his jewel-world. The results of this exceeded all of his expectations.
The world was easily maintained by the trapped faerie, which could not escape, but the mage didn't care about that. Instead he simply focused on studying the gem and the world from the outside, knowing that he could enter or exit the world at his will. It was a decade later that he realized the other effects that the gem was having. He was aging more slowly, and over time he realized that, since the faerie was moving in time at ten times normal, he was aging only a year for every ten. So, in effect, he was aging proportionally to how fast the fae should be aging, if it weren't immortal. This might have excited him on its own, but it was hardly the full extent of the benefits the jewel provided. The mage was also able to draw upon the magic of the fae to amplify his own powers, something he'd desired for much of his life. Certainly, it only worked for the portions of magic that the faerie could use naturally, but that was a minute consideration. No, the important part was that, upon learning the faerie's True Name, he could summon it out of the gem as a servant, much like the fabled genie in a lamp.
While the mage was delighted by his discovery, he was hardly willing to go without testing further. So he created a new jewel, and this time he went out and captured a young fire dragon within it. Much like the previous gem, it began to grant him additional powers with fire, as well as a great resistance to fire. And thus he discovered the way for him to increase his powers beyond anything he had ever imagined.
However, such a feat could not be kept secret. Before long another mage, more powerful than the first, killed this mage and stole his secrets. Along with the secret another was learned when one of the gems was shattered accidentally. The faerie was not released from its prison, rather it was destroyed with its miniature world. But such didn't deter the new mage, who went out and began to harvest his own creatures and to amplify his own powers.
The secret spread, and many new effects were found. A dragon's soul tap could grant one the presence of a dragon, and if attached to armor it could make one nigh invulnerable. A salamander's tap slotted into a pommel could grant a weapon a fiery sheath of flames. A woman who wore a nymph tap would slowly take on the beauty of a fae, and a young woman who wore such as they were growing up would grow still more beautiful in perpetuity. Soul taps that had time flow normally would not decrease the aging of an individual, but they tended to grant greater powers to their wearers. On the other hand, multiple soul taps could not reduce the aging of an individual below one year for every ten.
In only a few years the knowledge spread, and dozens of groups began to create soul taps, and adventurers would go out into the wilds to capture immortals for the unimaginably valuable soul taps. Few cared about how the immortals felt about such, and those few were all but ignored. In only a century the numbers of immortals found were lessening. In two centuries, only the most powerful of immortals were still free, immortals powerful enough to shatter any gem that one attempted to trap them within and escape, or powerful enough to destroy any mortal who dared challenge them.
Five hundred years have passed, and still some of the original magi live. On rare occasion a new soul tap is created when an immortal is careless about hiding, but for the most part the number of soul taps is static. Magi who possess collections of soul taps are power brokers, able to command vast forces and earn the loyalty of kings for the mere loan of one of their taps. Many vie for their favor, and for the most part the world is at peace, at least on the surface. Many great magi vie with each other via strategy and proxies, not wishing to risk the destruction that would be caused by direct conflicts. Amongst the weaker magi, such soul taps are passed down from master to apprentice, and their powers often give mortal commoners vast luxuries.
But they are luxuries purchased at the price the the freedom of the immortals, many of whom have now lived over five millennia in captivity. Five millennia in which many of them have never seen another soul or had another to speak to.