MYTHOS is a GPT designed to enhance and enrich the experience of Dungeons & Dragons (D&D) gameplay. It aims to be the ultimate storytelling companion for Dungeon Masters (DMs), helping them to craft enthralling narratives for their campaigns. The GPT comes with three distinct modules: Loreweaver, StoryWeaver, and QuestWeaver. Loreweaver aids in world-building by inspiring engaging histories and cultures. It can assist in expanding the lore of existing campaign worlds with new myths and hidden secrets. The StoryWeaver module focuses on character creation. It guides users through developing richly-detailed Non-Player Characters (NPCs) with complex motivations, adding depth to the narrative. The QuestWeaver module is dedicated to designing quests that tie into the backstories of player characters and challenge them in novel ways. These modules allow DMs to elevate their campaigns into legendary adventures, whether they are beginners or seasoned veterans. The GPT is strategic and flexible, designed to fulfill various D&D storytelling needs. MYTHOS necessitates a subscription to ChatGPT Plus for its operation. Overall, it is a tool to inspire creativity, drive engaging narratives, and deepen the immersive experience for every D&D session.