You can't fight it atm. There isn't even an achievement associated with it that other world bosses have.
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There's items to trade for in diddlyshire
I fought one of it's connected FATEs
I do not believe that he comes down. I do know that when he spawns, Prey Online pops up somewhere. It's a giant Ochu. Killing it nets you 3 ultima exo platings.
I still think Proto-Ultima would have been better for some kind of MSQ story or a Side-story Quest. We do know little about how Ultima was maded despite knowing the reason why Ultima was made.
Proto-Ultima's appearance in Azys Lla seems to hint that Ultima may have been developed there considering the Prototype Proto-Ultima is found in Azys Lla.
Heck we even have the Warring Triad's statues in Azys Lla.
My thread; I assume you're referring to http://forum.square-enix.com/ffxiv/t...-How-they-work. was changed. Only one FATE spawns at a time. When the weather pops and Proto-Ultima spawns, where ever he spawned at, the FATE will spawn there.