Everything looks correct, though I guess the river/lake biome could be problematic. Since they apparently kill people, they should be able to convert as well. As a test, you could give them a wetland biome to see if they can convert creatures that way.
That's actually a good idea. I shall try it, and then edit this post with the results.
Edit:
No dice. At most they kill one person, who has to confront them (instead of the other way around). They're always an enemy of every civ though. Sometimes they don't kill anyone at all.
I set the biome to ANY_LAND, and I'm giving it another go. 250 years, small world, high civ and site.
Edit 2: Electric Boogaloo
Again, nothing. Now it tells me that he began wandering a bunch of different biomes. He still doesn't seek out any fights, and he still doesn't kidnap and convert anybody.
Edit 3: Return of the son of Edit 2 - He knows what you did last summer
I dropped the difficulty of the fuath down from 10 to 1.
He now actively starts fights with everything from regular joes to other night creatures (which I'm ok with).
In the Early Spring of 48, Rulush Bentpolished abducted Sigun Canyonshark from Handlepolished and changed her into a monstrosity.
PS. These dudes are so hardcore they're devouring trolls in worldgen.
Anyways, it seems like night creature spouse converters work BUT they need to be [DIFFICULTY:1]
I'm not sure why this is, and it needs more testing to see if you can raise it any higher than that. But that's something I'll get around to another day, maybe.