Of those who visit the Grey Isles, painters, poets, writers, scholars, philosophers, playwrights, actors and muses are special targets for another form of horror: the Xanthic Sigil.
The lush, melodramatic atmosphere of this dreaded place is enough to fuel even the most sterile imagination. Those with a creative flair, however, will likely witness this phenomena and be forever changed if they allow themselves to be overcome by their own curiosity and obsession.
The Sigil will first appear to them in a subtle but eye-catching way: as a detail in a painting or sculpture, in an otherwise benign page of a book or as a piece of jewelry on a standout person at the palace.
Afterward, on their first night on the Isles, they will dream of the Sigil; it will appear important and portentous. The next day, they will see the symbol in other, similar places but no one else will notice or even perceive it at all.
If the individual inquires about the symbol, or begins to research it in the palace library, they will be at risk of obsession. They must pass a Wisdom saving throw or several frenzied hours will pass in their quest for knowledge and satisfaction.
The Xanthic Sigil will come to represent forbidden desires, unfulfilled promises, tormenting regrets and a pervasive, inexplicable angst. Others will notice that the character has unknowingly decorated themselves with the symbol somewhere on their person: drawn or painted onto their clothing or jewelry, usually.
The only way to overcome this obsession is to leave the Grey Isles for at least a week. Even while they’re gone, the fascination will persist and the Sigil will haunt them in other places. After a week of abstaining from this pursuit (requiring a Wisdom save each day), they will be free.
If left to pursue this emotional situation, they will find that the Sigil itself is a great source of power: it stands in for any foci needed for arcane arts of any kind, and replaces any required components for rituals. A character can even forego sleep to instead obsess over the Sigil: while they will heal one less Hit Die per long rest, they only require half the normal minimum number of hours needed.
If 2 weeks go by for a character who deliberately obsesses over the Xanthic Sigil, they will be visited by a cloaked and masked figure (that no one else can perceive in any way, even through magical means). This being will follow and watch them for hours, causing great disquiet and fear in the character (disadvantage to all checks requiring mental or emotional focus). After 1d6 hours, the player shall be told by the Referee that their character is suddenly keenly aware of their dark fate: they are doomed.
The character will die of terror. In a fortnight they will arise as a Wight of Kharkossa, in another plane, to serve in a nightmarish court for eternity.