What is "nostalrius"

Nostalrius was a private World of Warcraft server created in 2015. The server featured a version of the game called "Vanilla", or, one of the first versions of the game (Version 1.12). The Server was shut down on April 10, 2016 for copyright infringement on Blizzard Entertainment. It was one of the most successful private servers to date, with over 800,000 accounts registered and 150,000 active accounts. Since the conclusion of Nostalrius servers, a discussion about the possibility of Blizzard opening "Legacy" servers has begun.