Game Mechanics Event bosses

Many an mmo have an event mechanic where there is a boss that has seemingly endless health, and the more damage you do to it, the better rewards you get when the event is over. With this idea in mind, you could have a dern beast or somthing as a boss, and the players could fight it and get rewards, and technically the boss isn’t dead so it could come back in a similar event. It could have different bosses, and when the event is over the players could reap their rewards depending on how much damage they did. This may be difficult with multiple people, but someone smarter than me can probably think of a solution for that.

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This is different event from festivals, just to be clear.