This game's categorization of "raids" is fucking retarded and extremely confusing. Let me break this down for you. Crystal Tower, Mhach, Return to Ivalice, and YoRHa: Dark Apocalypse are called ALLIANCE RAIDS. Alliance Raids are easy, casual 24-man raids that take about 30-60 minutes to clear depending on the group. These are for casual players.

The Binding Coil of Bahamut (Normal/Savage), Alexander (Savage), Omega (Savage), Eden (Savage), Unending Coil of Bahamut (Ultimate), The Weapon's Refrain (Ultimate), and The Epic of Alexander (Ultimate): These are the game's hardcore raids. These will NEVER be mandatory. They consist of 8-man parties that face extremely tight mechanics and dps checks for the game's rarest loot. All the Normal versions of these raids, with the exception of The Binding Coil of Bahamut, are casual content.