A grey, spotted feather

As a predatory bird, the ysol glides through the forest at dawn, looking at foxes, rodents or insects to feed on. Despite resembling a common forest owl in appearance, some differences can be drawn between those two. The ysol is generally slightly bigger when fully matured and contrary to other owls, it is incapable of producing vocal sounds, instead, it is communicating solely through movement. Ambushing its prey with striking precision as it darts down, piercing the victims flesh with giant, sharp talons. Despite its size, this bird manages to get close to its targets due to the unique feathers it grows as it matures.
These feathers silence the wingflap, effectively enabling the ysol to fly silently. Combined with it's white and grey coloration, this predator is perfectly camouflaged and rarely seen in the wild. Sometimes however, skilled rangers and hunters managed to catch a live speciment. They have since been bred in captivity, bought by nobility and rich merchant guild leaders as additions to their personal collection of exotic animals.
While the animal itself is an expensive commodity, feathers of the beast can be commonly found as it replaces and regrows them often throughout its lifetime. Simple in appearance, a grey feather with white spots, it is often overlooked by travelers. They are used as part of decorational items or to feather arrows but a small secretive group is using this item as a mark of membership.
Wearing them on their person, leaving them behind after having finished a task, this organizations spirit draws many parallel to the behavior of the ysol. Approach the target unnoticed, ambush and grant them a swift death, fade away into the night, leaving only a feather behind as a warning to those who might oppose you.