Occasionally referred to as Lightbane, the Ring of Shadows is a tarnished silver band that feels chilled and greasy when first donned. Runes of power etched upon it can be seen to glow a noticeable blue when the ambient temperature is below 0 degrees Fahrenheit (-17 Celsius for my metric readers).
It radiates necromancy and illusion magic under a Detect Magic spell or its equivalent.
Attunement: this is a permanent loss of 1d4 Constitution points as it binds itself to the flesh of the fool being that slips it upon their finger. Even if the ring is successfully removed, those Constitution points are gone.
Passive Powers: once attuned, the wearer gets Advantage on all hiding-related checks. In addition, shadows shift about the wearer, granting them both an AC bonus as well as Advantage on intimidate checks in dim light.
Activated Powers: the wearer can shadow walk, at the cost of a Constitution save. Failure to save (DC 12) means an exhaustion level is acquired upon exit from the shadows. The wearer can bring others with them on this shadow walk, if all involved are holding hands. The ring-wearer must make saves for each additional person on the walk; these saves increase by 2 each time, so taking three people on a shadow walk requires four saves (the wearer and three companions) at DC 12, 14, 16, and 18.
Shadow Walking allows quick transit, up to 50 miles per round moved.
In addition, the wearer can sever a sentient being's shadow, resulting in the being going increasingly mad as their shadow darts around them and taunts them from a distance, because it has limited freedom and sentience. Others can plainly see the shadow moving of its own accord and act accordingly (probably in a fearful fashion). This is a slow process, and mechanically, this madness manifests as the being refusing to rest until it reattaches the shadow (impossible without magic), gaining a level of exhaustion per day until death, spending all of their energy focused on the shadow, to the point of not eating. Then they rise as a free-willed shadow of maximum hit dice.
Manifestations: the wearer's breath is always visible, as if breathing out on a chill frosty morning. This is noticeable to anyone with eyes. In addition, shadows within 30' of the wearer dance and twist of their own accord, as if trying to attach themselves to the wearer.
Drawbacks: While living, the wearer suffers Disadvantage to all rolls in daylight. When the wearer dies, they become a shadow with maximum hit points and twisted memories of the past, leaving them convinced that their former allies (the surviving PCs) were at fault for their death and undeath. As such, they can be used as a source of information about said allies, but only the most potent of necromancers can bind them long enough to gain that knowledge.
The ring can only be removed by severing the living finger or removing it from the shrunken husk of a corpse. Furthermore, those who die while wearing the Ring of Shadows can be neither resurrected nor reincarnated under normal circumstances.
Research suggests that traveling to the Underworld and making a trade with the God(dess) of Death can restore a person lost to the Ring of Shadows to life, but the same research suggests that the cost is more than most mortals are willing to accept.
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For all its over-reliance on rules and books and more books, 5e's exhaustion is a lovely mechanic.
Even more lovely is the idea of advantage and disadvantage. I daresay this is both elegant and easily portable as a houserule into other systems.
