do you know the Ur-Dragon from Dragon's Dogma? i would like to have something like that.

a giant super boss spawns once a month or whatever and he has a huuuuuuuuuuuuuge amount of HP. it will take a lot of time to kill him, mabye days. you can go in and out of the fight as often as you wish and you will get rewarded for your contribution after the boss is dead. the more you help to kill him, the better is your reward. and if you are there in the moment he dies you will get an extra reward too.