According to these legal documents Blizzard is suing Valve to prevent it from trademarking DOTA name. Blizzard claims that the DOTA name has been aligned with the Warcraft series for the past seven years. Defence of the Ancients was originally created for Warcraft III as a mod.
“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,” claims the suit, “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.
“Valve has no right to the registration it seeks. If such registration is issued, it not only will damage Blizzard, but also the legions of Blizzard fans that have worked for years with Blizzard and its products, including by causing consumers to falsely believe that Valve’s products are affiliated, sponsored or endorsed by Blizzard and are related or connected to Warcraft III.”