The Forgotten Prince

Once there was a prince of a grand kingdom, the youngest of eight sons. His brothers were celebrated as heroes of the realm, but whatever the youngest prince did, his deeds were never noticed; the shadows cast by his brothers were too long. Even when he saved the kingdom from a cruel dragon, the people assumed one of his brothers had done the deed. But in that dragon's lair, the prince discovered a strange sack. Reaching inside it, he found a toy he'd loved as a child and long forgotten about, a comb lost by his sister, and a map of a nation that no longer exists. Curious, he climbed into the sack and found a castle within it, filled with forgotten wonders. No one ever noticed he was gone, and no one remembers his name or his deeds. But the Forgotten Prince still dwells in his Castle of Lost Things. When you forget something you love, or when something you need is nowhere to be found, it's because the Forgotten Prince has taken it for his hoard.
When a sock goes missing or a tool is misplaced, people curse the Forgotten Prince for stealing it away. When someone forgets what they were about to say, they might remark "The Prince has taken my thought." Needless to say, his reach is not truly as wide as such stories make out; but the Forgotten Prince and his minions do indeed steal unappreciated things, both material items and even ideas themselves, keeping them in the vaults and library of his Castle of Lost Things. Often such thefts are never noticed, but he might take things that are quite valuable if he believes them to be unloved or unappreciated by those who have them. Sometimes this is done to teach a lesson, how the loss of a simple and undervalued item or idea can have disastrous consequences. Battles have been lost and cities fallen because of the Prince's thefts—though often people fail to appreciate the lesson, as they never recall what was lost to begin with.

The Prince of All Things Forgotten and Unloved. The Forgotten Prince dwells in the Castle of Lost Things in Thelanis. His attendants are sly and cunning fey, along with mortals who were unappreciated in their mundane lives. Some say that the kenku are servants of the Forgotten Prince who fled from his service—that he stole their voices as punishment for their betrayal, but that a kenku can earn it back with clever deeds. True or not, the Prince has followers scattered across Khorvaire—thieves who sacrifice unloved objects and undervalued secrets to the Prince in exchange for his guidance and gifts. Followers of the Forgotten Prince rarely work together, but they can be found in many positions of influence. There are elves in the Shadow Houses who receive visions from the Prince when they trance, and changelings among the Tyrants of Sharn who engage in more subtle thefts on behalf of the Prince, stealing hearts or faces. While his thefts often go unnoticed by the greater world, the Forgotten Prince also takes pleasure in seeing celebrated people whose reputations are undeserved brought low, and his agents may act to expose such poseurs. Adventurers could also be forced to deal with the Prince if he has something they need—perhaps an artifact or secret long forgotten, now sealed away in the vaults of his dark castle.
The Prince as Ally or Patron. The Forgotten Prince may reach out to people who have themselves been forgotten or unappreciated, provided he sees potential in them. Sometimes he brings them to the Castle of Lost Things; in other cases, he offers these agents purpose, rewarding them for their service with gifts or secrets. The Wayfarer background is an excellent option for someone forgotten by the world but chosen by the Prince, while the Hermit background could reflect someone who spent much of their life in the Castle of Lost Things. The Forgotten Prince could be an Archfey Patron for a Warlock, or a Hexblade Warlock (Xanathar's Guide to Everything) could also receive their weapon as a gift from the Prince; what is the forgotten history and destiny of this weapon? The Forgotten Prince can also be an interesting patron for a Bard of the College of Lore; the Prince values secrets no one else considers valuable, and the Cutting Words feature can be depicted as stealing opportunity from a foe—a trick taught by the Forgotten Prince. The Forgotten Prince could teach a Rogue the talents of an Arcane Trickster or the skills of a Thief, provided they are willing to use those skills on his behalf.
Missions issued by the Prince typically involve the theft of an object or a secret. These things may not be seen as valuable by the people that possess them, and such a task could easily be added on top of the mission for an existing adventure—"While you're in Dreadhold, steal the ring under the warden's bed." The Prince could teach his agents a ten-minute ritual which allows him to pull the requested item into Thelanis (this only works with objects the Prince wants, so it's not a way to dispose of just any item). The Prince may also task agents with actions to expose or bring down people whose reputations are undeserved, or to assist those who are unappreciated. Note that good and evil don't enter into these judgments; the Prince could call on agents to bring down someone who's using their undeserved reputation for a good cause, or to help an unappreciated serial killer!
The Prince as an Enemy. The simplest way for the Prince to enter a story is for one of his agents to steal something that the adventurers need, or to take actions that cripple an organization or patron that the characters work for. This ties to the idea that the Prince may target someone with an undeserved reputation even if they are using it to do good; the adventurers themselves could be targeted if they acquire fame that the Prince considers unearned. Or the characters might discover they need something stolen long ago by the Prince—or an object they've recently acquired and desperately need has gone missing, and no one else seems to remember it. Characters could also be set against the Prince by another immortal patron; perhaps the Prince has stolen one of the memories of the Forest Queen, and she wants it back.
The Power of Secrets. A defining part of the Forgotten Prince's story is that he himself has been forgotten. Should a character somehow learn the true name of the Forgotten Prince during their adventures, they have Advantage on saving throws against spells that he casts.