You need to complete it to do his first fate, and you need to do his first fate to do his skill unlock fate.
Topic on Talk:Sandalphon (Event)
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His third skill needs completion of story chapter 54 to unlock.
And after the change to his Fate Episode unlock, you now need to do this on top of clearing the following side stories:
Festival of Falling Flame The Cowardly Hero and the Captive Princess Blade of the Young Champion What makes the Sky Blue
..in order to unlock it.
In order to do his fates, you need to finish WMTSB part 2 and to unlock that you need to do part 1, before part 1 you need to do other side stories and they have their own requirements. There's no need to include each one of them since they're on WMTSB part 1 page which part 2 links to.
I know, I wanted to note that the requirements changed at some point.