On the left is normal Hau from Pokemon, on the right is my interpretation of what would happen if he was stung by a Nihilego and what the fusion would look like.

Basically his fusion is caused by an infection where he was stung by a Nihilego when he accidentally fell into an Ultra Space portal while the protagonist was off stopping Lusamine. All was good until Hau started experiencing memory loss and his entire body started changing to look like a Nihilego Fusion. The Nihilego that stung him happened to be the King of the Ultra space and was dying as a result of Lusamines tampering with its home. So eventually Hau's infection took him over and he was crowned the new king of the Ultra space, it came with power and a chance to beat the protagonist. Which he did... with relish. Then he started his genocide on Alola and imprisoned all characters who could stand up to him but left MeleMele alone for the most part because he still had a soft spot for his grandfather Hala. Which became his unknown undoing since Hala now leads the resistance with Gladion as a spy in the Aether Foundation and Moon as a spy in Hau's castle since she was now a maid there.

Yet ten years after his genocide on Alola he met a young girl who he took a liking to... who then kicked his ass six ways to Sunday.

If you want to know the story look at my Rewritten version of my book Defender where my OC Shard travels the Alola Region to end Hau's reign.

Progress shots:

The Nihilego Fusion parts were hardest to do since it took ages and sometimes my iPad shut down on me and I had to start again.