Secret of Phantoms' Cloister is an adventure module from the RPGA Legacy of the Green Regent campaign. It was never released to the public.

In the last adventure, the party entered the Mind of Karsus, where an unspoiled version of Phantoms' Cloister, called the Monastery of the Fallen One, exists. It is populated by the shades of monks who worshiped the momentary god but now desire only release from their extraplanar prison. Along the way, the party can rescue a relative of the High Lord of Loudwater, Nardine Twohands.

Secret of Phantoms' Cloister was never publicly released. Information about the content of the adventure is taken from other modules that were released.