Blizzard has filed a case with the US Patent and Trademark Office, claiming that Vale has no right to trademark "DOTA" for exclusive use in the world of gaming.
The company's chief argument rests on the fact that Defense of the Ancients, originally a Warcraft III mod, has become so associated with the company that Valve will be unfairly stealing the reputation and brand identity of a product they did not create.
"By virtue of that use, the DOTA mark has become firmly associated in the mind of consumers with Blizzard, including to signify a highly popular scenario or variant of one of Blizzard’s best-selling computer games, Warcraft III," the statement reads. "Over the past seven years, the mark DOTA has been used exclusively in connection with Blizzard and its products, namely Warcraft III.
"By attempting to register the mark DOTA, Valve seeks to appropriate the more than seven years of goodwill that Blizzard has developed in the mark DOTA and in its Warcraft III computer game and take for itself a name that has come to signify the product of years of time and energy expended by Blizzard and by fans of Warcraft III."
Of course, the picture is hardly as clear as Blizzard's legal team wants the authorities to believe. Blizzard didn't create DOTA's brand identity any more than Valve did, and just because the mod was built atop their game doesn't grant them clear-cut ownership of the trademark.
It's worth noting, though, that Valve has gone to great lengths to avoid using both full capitalization and the mod's complete name, Defense of the Ancients, in any and all promotional materials for Dota 2. They're well aware that they're attempting to create a sequel to a game they don't explicity own the rights to, and the fact that the broader picture is so muddy might throw a wrench into their trademark filing.
Trademark or not, Dota 2 is currently in beta and is expected to launch publicly later this year.
