Obituary Sparks

Those aren't fireflies, fairy dust or floating festive lights. Yes, they are sparks, but not the kind you are thinking.
They are a ghost that loves a cute sad ending. It's, their favorite meal, thought they can survive with any form of decay: dead tissue, rotting fallen trees or dead animals... even the shards of the loss of someone who died.
When they can't find someone who is constantly suffering by memories of someone who died, they follow around people with debilitating or degenerative diseases. The rest of the time, they hunt in The Forest, especially in dangerous or constantly changing areas.
While they eat, people around them is basically forced to revive the every sad detail they ever knew about the life of a loved one who died. The consequences of that experience vary depending on the person. Some are happy to remember, even if they can't stop crying. Other feel despair or rage, or even fear. For some, it's enough to allow them to revive the good times too, since the thoughts of one event leads them to another.
Someone took one to the Underground Library to gather the information to tell the story of an interesting person they knew. The idea worked so well, that they continued to invite these ghosts, even after they discovered that the library's magic will literally murder the closer person to feed an obituary if it is hungry.

Origin

These ghosts' core is formed by the shards of lost happiness, and the other shards involved are usually related to human connection, like the loss or relatives, supporters and loved ones.
The most common trigger for these ghosts is the influence of a powerful need to keep a memory alive, but there have been cases when they are created for the shard's exposition to the determination to recover something.