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Ragnarok is a historic term for an ancient battle that takes place in Norse Mythology. It cannot be copyrighted in any way.

the name Ragnarok is not copyrightable, yes, true, copyrights are for completed works, not names - but did you register it as a Trademark? That is where you can get into legal issues dealing with intellectual property. Individual names/words can be trademarked on a per category basis.