The Crimson Squad was an elite group intended to surpass the Crusaders introduced in Final Fantasy X-2, whose best members were to be assigned leadership of various groups of Crusaders across Spira. Notable aspirants for the squad include Nooj, Baralai, and Gippal, with Paine as the group's recorder. The name is likely derived from Crimson Blades, the original name of the Crusaders.
Maester Wen Kinoc assembled the Crimson Squad under the guise of fulfilling the society's demand for a stronger army, but the true reason was to privatize the Crusaders by inserting his hand-reared Crimson Squad members. Feeling threatened by the newly appointed Maester Seymour who was gaining power, Kinoc set his eyes on the Vegnagun, an ancient weapon shrouded in mystery to even the Maesters of Yevon. Rumors circulated of a vengeful presence inhabiting the Den of Woe below Mushroom Rock Road with reports of people seeing the illusion of a "gigantic machine" within intrigued Kinoc.
For the Crimson Squad recruits to be officially assigned as members, they were ordered to investigate the mysterious Den of Woe, where unknown to everyone the spirit of Shuyin had lain for a thousand years. Kinoc sent the Crimson Squad into the den to collect information on the Vegnagun on the same day as Operation Mi'ihen so any casualties would go unnoticed, as he knew Operation Mi'ihen would be a massacre.
Bitter and consumed by grief and despair, Shuyin's emotions possessed the Crimson Squad candidates who turned on one another, leaving all but Nooj, Gippal, Baralai and their sphere recorder, Paine, dead. Logos and Ormi, working under Kinoc, discovered that there were bodies missing, and after the survivors escaped from the den, Yevon forces attempted to kill them, as no information about the Vegnagun was gained. The failure placed Kinoc's position in jeopardy so he ordered the extermination of Crimson Squad survivors who knew the truth—that Kinoc's orders had been the cause of the deaths.
Kinoc ordered the Den of Woe sealed with magic, but given the chaos brought on by the collapse of the Church of Yevon, a rumor circulated that it had already been sealed at the time of Operation Mi'ihen. Sphere recordings made by Paine, called "Crimson Spheres", become scattered throughout Spira, but if Yuna's party collects all of them they gain access to Den of Woe and uncover the truth.
"Crimson Squad", also known as "Party", is a music piece which includes vocals and drums. It plays whenever Crimson Sphere recordings are watched and acts as location theme for the Den of Woe.