The cold war standoff between two of the most popular online games in the world just got heated. PUBG Corp, the creators of PlayerUnknown’s Battlegrounds, have filed a lawsuit against Epic Games, developers of Fortnite, for copyright infringement. As PUBG Corp is based in South Korea, the suit was filed in the Seoul Central District Court.

As is the case with ongoing litigation, neither side is offering commentary beyond PUBG Corp telling Bloomberg News: “This is a measure to protect our copyrights.”

The two companies have previous ties via the Unreal Engine. Epic licensed the engine to PUBG Corp to produce PlayerUnknown’s Battlegrounds.

PUBG originally launched in Spring 2017 via Steam Early Access, officially releasing in December of last year. It’s 100-person free-for-all deathmatch (“Battle Royale”) was an instant hit, selling tens of millions of copies and becoming a primary destination for Twitch streamers and YouTube personalities.

Then came Fortnite. Fortnite initially launched as a co-op tower defense game, also in Early Access, last summer. It was mostly middling, until Epic Games released a free-to-play Battle Royale spin-off last fall. Fortnite Battle Royale uses the exact same Battle Royale gameplay hook as PUBG – dropping 100 players onto an island in a one-person-standing deathmatch.

In the months since, Fortnite Battle Royale (oft shorted to Fortnite) has become a gaming phenomenon, rivaling the likes of Minecraft and Pokémon GO.

Fortnite Battle Royale had three distinct advantages over PUBG – it’s free-to-play, available on consoles (PUBG has since released on Xbox One via Xbox Game Preview), and it’s rated T for Teen with a more palpable cartoony art style. Fortnite Battle Royale also features the free-form base-building featured in the Fortnite tower defense game, providing a unique twist on the genre.

The lawsuit doesn’t make PUBG look very good. PUBG’s huge numbers have been steadily declining in the months since Fortnite’s release, though it still boasts over a million concurrent players on Steam at any one time.

These days an online game’s success is often measured by the amount of viewers on Twitch. In this case Fortnite is the clear winner as the most watched game on Twitch. One of the most popular streamers, Ninja, infamously switched from PUBG to Fortnite earlier this year.

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Eric has been writing for over eight years with bylines in Dicebreaker, Pixelkin, Polygon, PC Gamer and Tabletop Gaming magazine, covering movies, TV shows, video games, tabletop games and tech. He reviews and live streams D&D adventures every week on YouTube. He also makes a mean tuna quesadilla.