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On January 9th, a lawsuit was filed in New York Federal Court by NCSoft alleging that TERA Korean developer, BlueHole Studios, and US publisher, En Masse Entertainment, "makes use of NCsoft's trade secrets, including documents, source code, artwork and know-how related to the development on Lineage 3, and copyrighted works, including concept art created for Lineage 3."
While this is a new legal proceeding in the US, similar cases have been adjudicated in Korea between NCSoft and BlueHole Studios resulting in both criminal and civil convictions against BHS. Both decsions have, however, had portions overturned and remain in appeals courts in Korea.
Whether or not the complaint will have an impact on TERA's North American release date of May 1st remains unknown though the conventional wisdom says that NCS will file some sort of injunction against publication of TERA on May 1st.
En Masse's community manager, Evan Berman, had this to say on the official NA TERA forums:
Evening, folks. We're aware of this. It's our policy not to comment on legal matters.
We'll keep you posted as the story develops. In the meantime, read the legal documents on the New York Federal Court docket.