Mordenkainen Presents: Monsters of the Multiverse
The shadar-kai are bound to the Raven Queen, cursed to forever serve her in the Shadowfell. They dwell in places outside the Fortress of Memories, usually too terrified of the place to enter it willingly. In their communities they reenact their old rituals and ceremonies, in a pale imitation of the days when they dwelled in the life and light of their now-lost kingdom.
When shadar-kai are in the Shadowfell, their bodies and faces are old and withered, displaying the full effects of the terrible magic that stripped them of their former elven beauty. To hide their visages, they often wear masks made of metal or wood, but even these coverings are melancholic in appearance. When shadar-kai are sent away from the Shadowfell to do the Raven Queen's bidding, they take on youthful features similar to those of other elves, although their skin remains deathly pale.
The shadar-kai know that when they die, the Raven Queen captures their souls and returns them to the Shadowfell, where they are resurrected to serve her yet again. Thus, they consider death to be a temporary condition, and many shadar-kai care little for the physical shell they currently inhabit.
Shadar-kai know that those who come willingly to the Raven Queen's tower are there to beseech her for something, and thus they try to prepare such visitors for what they will face. The queen's servants talk to any inquiring adventurer about the gravity of emotion, how sorrow weighs on the soul as it travels through the Shadowfell, and how best to persevere in the Raven Queen's test.
When the Raven Queen sees a soul or a piece of information she wants, she sends her ravens to alert the shadar-kai. Her minions then put their trust in these cryptic, cawing guides to lead them to where the barriers are weakest so they can then slip across planes to their destination. Once at their destination, the shadar-kai watch and wait, looking for the tragedies their queen wishes them to collect. Sometimes they are small: a spurned lover, a lost item, a betrayal. But some tragedies are much graver: a murder, a war, a diabolical bargain. To bring back a trinket for their queen, the shadar-kai use their shadow magic. If a target is living, they magically infiltrate the person's mind and excise the desired bits of emotion, or if the target is close to death, the shadar-kai capture the whole soul to bring back to the Raven Queen.
Shadar-kai are very interested in the magical silt at the bottom of the River Styx that holds the memories and identities of lost souls. Any adventurers who travel to the Nine Hells to procure a vial of this powder will likely draw the attention of the shadar-kai, who will attempt to steal or barter for it. Adventurers might also bring a bit of the sediment as a gift to the Raven Queen. What she would give in return is never known ahead of time, but her boons come in many wondrous forms: the restoration of a lost soul, the rediscovery of a missing memory, or a glimpse into the forgotten knowledge of the ancients.