A Mourner is a creature found in the Tempest in Wraith: The Oblivion. Although many consider them one of the many kinds of Whistimmu that create hazards throughout the Tempest, the truth is much more sinister and sad.
Mourners, at first glance, resembed the cloaked figures of the Ferrymen. Instead of bearing their lanterns and scythes, however, Mourners wear masses of heavy chains. Their faces are unseen, shrouded in the darkness of their cloaks. They will always travel in packs, with all Mourners in a particular group chained to the most powerful among them. Unlike many of the creatures of the Tempest, the Mourners always seem to have a specific target when they appear, marching from the Tempest into the Shadowlands. The Mourners hunt down a specific wraith, group of wraiths, or even, as legend says, living humans. Once a Mourner touches a wraith or other being, that person is marked. Even if they miraculously evade the Mourners that time, the Mourners will be back and will keep trying until the target is among their numbers. Any who dare fight them, whether the Mourners are interested in them or not, will be wrapped in the sentient chains they bear and dragged into the Tempest. The next time they appear, they will be a Mourner themselves.
Their resemblence to Ferrymen is no coincidence, as the Mourners were the product of accidental experimentation by the Boatmen's Society. During the nineteenth century, the Fetters of the Deathlords were mysteriously destroyed. Unaware that the actual perpetrator was Charon himself, the Ferrymen, specifically the Society's College of Inquiry, decided the Ferrymen were also vulnerable to such attacks and decided to find a way to strengthen the protection around their Fetters.
The seven members of the Society decided the best way to protect the Fetters would be to draw them through the Shroud somehow and transport them to Dis where they could be locked away. It sounded like a good idea on paper, but the consequences of a wraith coming into contact with their own Fetter in the Shadowlands were not well-thought out. With a combination of Usury and Embody, the Society managed to draw one of their own Fetters through the Shroud. However, the act of drawing it caused a weird resonance in the Shadowlands and began transforming five of the Society members. Unable to stop their escape, the remaining two Society members managed to contain the abilities of the newly-transformed beings with Puppetry-like abilities, although they were unable to capture them.
Even towards the end of Stygia, much about the Mourners remained unsolved. All of the Mourners were drawn to Stygia following the return of Charon and the invasion of Coldheart. However, the Mourners went after Spectres instead of wraiths, assisting Stygia's army in bringing down Coldheart's troops. What happened to them afterward is unknown, save that many lasted through the battle.