From the Introduction: Matter Undermined is a standalone encounter designed for five 19th to 21st-level adventurers. An ancient temple has been unearthed, releasing the power and influence of Ythilus, the elder brain from a destroyed illithid empire, which has corrupted the minds of nearby humanoids in an attempt to regain power. The players must descend into the temple and fight off the mindless thralls and kill the elder brain, all while maintaining their sanity