The Chaos Shrine from FFI. We already use the Warrior of light motif and it can easily be split into 3 different raids. Garland being the final boss of the ruined shrine version, the idea of a temporal rift allowing you to go back in time to the original version of the tower allowing you to fight the four elemental fiends and a final part that is the true chaos shrine with chaos being the final dungeon boss. Could build off the idea a lot more but a bit tired right now lol.

An honorable mention would be Pandaemonium from ffII