Blizzard has filed a Notice of Opposition with the US Patent and Trademark Office in a bid to block Valve's registration of the Dota trademark.
Valve sought to make the Dota trademark its own last year in anticipation of releasing its online game Dota 2.
Yet the Dota name had been used for years in various fan-made modifications of Blizzard games.
Executives at the World of Warcraft company have previously made public their complaints about Valve’s trademark attempt, but in November it pressed ahead with a lawsuit.
"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 company said.
Valve has opposed the complaint and wishes to have its trademark enacted.