Yuzu is a free emulator released in 2018, months after the Nintendo Switch first came out. The same lot who made Citra, a Nintendo 3DS emulator, made this one. Basically, it's a piece of software that lets people play Nintendo Switch games on Windows PC, Linux, and Android devices.

It also runs on Steam Deck, which Valve showed -- then deleted -- in a Steam Deck video clip.

Emulators aren't always illegal, but nicking games to play on them is. But Nintendo said in its lawsuit that there's no way to legal way to use Yuzu.

Nintendo argued that Yuzu runs codes that "beat" Nintendo's security measures, including decryption using "a nicked copy of prod.keys." "

Without Yuzu's decryption of Nintendo's encryption, pirated copies of games could not be played on PCs or Android devices," Nintendo wrote in the lawsuit.

As to the alleged damage caused by Yuzu, Nintendo pointed to the release of The Legend of Zelda: Tears of the Kingdom. Tears of the Kingdom leaked almost two weeks before the game's May 12 release date. The nicked version of the game spread fast; Nintendo said it was downloaded more than 1 million times before Tears of the Kingdom's release date. People used Yuzu to play the game; Nintendo said more than 20 per cent of download links pointed people to Yuzu.

Though Yuzu doesn't dish out pirated copies of games, Nintendo repeatedly said that most ROM sites point people toward Yuzu to play whatever games they've downloaded.

Nintendo said it's spent loads of dosh to stop the illegal copying, marketing, sale, and distribution" of its Nintendo Switch games. It says that Yuzu earns the team €26,000 per month on its Patreon from more than 7,000 patrons.

Nintendo said the company has earned at least €43,000 in paid Yuzu downloads. Nintendo said that Yuzu's Patreon doubled its paid members in the period between May 1-12, when Tears of the Kingdom was released. Nintendo is asking the court to shut down the emulator and for damages.