A Boss that can only be fought after beating the other bosses

A Structure spawns in every world but will only open after the Enderdragon, Wither, and Elder guardian have all been killed at least once. This structure would be linear and the final room would hold the Final Boss. On the way to the boss you have to fight mini bosses that are enhanced versions of regular hostile mobs. I think the main reward of conquering this dungeon would be hints at the lore of minecraft and what happened in the world before the player arrived.