The spire: a proper 1.20 structure

Ever wanted an optional challenge that made Minecraft more interesting? Ever thought the current bosses and the single "raid" event was rather redundant, and not very rewarding? Summon forth the spire, a new event-boss hybrid where the player must defend a tower containing a dark crystal of unknown name or origin. In the plains, Badlands or Mountains, a large 3 block wide, 7 block tall structure will spawn with a Guarded "Shadow crystal" at the center, reminiscent to the end crystal.

Your job is to keep it contained. Once you get close, the event will start, and a horde of mobs will be summoned in waves, ranging from zombies & skeletons to witches & creepers, who all wish to destroy the container. If the player succeeds, the shadow crystal will dissipate, and the "Magic container" can be repurposed as a monster spawner that spawns mobs based on what drops you put in. However, if you die or if the mobs deal enough damage to the container, the shadow crystal will be released, which will act as an infectious item that can only be grabbed if you have armor on, as otherwise you'll be given the wither effect from getting close or staying on top of the "murk", a new trap block that could slow down & wither mobs slowly. 

Generally a rewarding structure to come across, and could also tell a story or give a chance to build your own lore surrounding this ominous structure.