Nighthaven is a realm created by Elysium-Project. Their realm is a blizzlike Vanilla Private Server that Elysium advertise as a Legacy Project, with focus on the blizzlike part.

Elysium-Project has previously had a lot of history, especially about the owners, and are known for a quite poor reputation.
The population on Nighthaven is not as large, as it used to be. You can expect around 200-500 players online on the realm.
There aren’t too many servers for this expansion with a larger population than Elysium’s Nighthaven.

The more recent feedback is that Nighthaven has gotten more stable, and has a respectfully playerbase, making it a possible choice when searching for a blizzlike realm.