Within the wider King in Yellow mythos there are many references to fictional and real people, places and concepts that the party discovered in their investigations into Alexander Roby and The King in Yellow.
Yhtill is one of three cities in the land of Hastur (though this name wasn't mentioned in the play as performed), along with Alar and Carcosa. Yhtill sits on the lake of Hali. It has a hereditary monarchy descended from its first king Aldones (Yhtillic for 'father'). The current ruler of Yhtill at the time of the play Carcosa, or The Queen and the Stranger is queen Cassilda.