Remnants

Fair Folk like to boast about being immortal, but they do not live forever, and they can be slain by violence.
Fair Folk also eventually grow old, though it takes a very long time for most with most of them having life spans between 250 and 2500 years old. Technically they do not "die" of old age, but they do cease to be.
During a Fae's "twilight time" a Fair Folk will lose his or her mental faculties while physically mutating, transition from one of the Fair Folk into that of a Remnant. Remnants take many forms, plant, beast, or even a landmark.
Remnants are often extremely strange, such as a perpetually flaming willow tree that is always making crying noises. Whatever form they take, Remnants have a symbolic connection to the Fair Folk they once was. Some Remnants are dangerous, some are beneficent, and many are merely benign oddities neither harmful nor helpful.
Monogamous Faerie couples often go into their twilight together and merge into a single dualistic Remnants. Likewise, twins are very common among the Fair Folk become merged Remnants when they die.
Some Fair Folk are comforted by the notion that a part of them will always live on, but others view the mental and physical mutations as terrifying. Fair Folk are a proud people, very few Fae will admit they fear becoming Remnants.
Certain Remnants are considered blessed and some are considered cursed, and the Fair Folk have many superstitions about what actions in life lead one into becoming a pleasant reincarnation or a foul one.

Remnant Management

It is generally taboo for one of the Fae to harm or destroy a Remnant, especially if the Remnant used to be a friend or family member. This is tantamount to a mortal grossly violating a corpse. Most Fair Folk will work to accommodate a nearby Remnant, even if doing so is inconvenient, but they will reluctantly destroy a Remnant that is overtly dangerous to Fae.
When Remnants appear in the mortal plane, many Fair Folk will try to relocate them back to Fae Home for the Remnant's protection though they can be hypocrites if a Remnant is highly dangerous. In such a case, Fair Folk will often deliberately push such Remnants into the mortal plane because they would rather not violate the taboo of not harming a Remnant when they can pawn the problem off on mortal adventurers (and maybe kill a few mortals they don't like in the process).
Remnants, as well as common Sprites can be destroyed by mortals and processed into reagents.
For most mortals, it is very difficult to tell the difference between a Remnant and a Sprite since the most Fair Folk try to hide the nature of their twilight time from mortal kind so even many Fae scholars have never even heard of "Remnants." 
A great many Fair Folk are completely indifferent to the slaughter or domination of Sprites but will go to great lengths to enact on vengeances who dishonor a Remnant. This of course makes the Fae seem even more fickle to mortals than they already think they are.

What about the Ones who don't make it to old Age?

Some, but not all Fae slain by violence leave something behind.  A "remnant" with a small "r" rather than a big "R."  Lesser remnants might a small flower bush, a large mushroom, a rodent sized animal, or a large tooth.
These lesser remnants sometimes have minor magical properties but they are often little more than harmless curiosities.  Some Fair Folk will try to collect these lesser remnants to stage a funeral with but many others view them as wholly inconsequential.
Fair Folk that die violently under highly emotional and/or supernaturally unusual circumstances occasionally leave behind Undead Remnants.  These creatures will eventually get their own article, but needless to say they are terrifying as they are rare.

Genetic Ancestor(s)

• Fair Folk

Geographic Distribution

• Fae Home