Activision acquired RedOctane in June 2006, gaining control of the Guitar Hero franchise.
RedOctane began as a company focused on gaming accessories and became closely associated with Guitar Hero after publishing the original game and producing its guitar-shaped controller. Harmonix developed the first Guitar Hero, but RedOctane’s publishing and hardware role made the company central to the franchise’s early success.
Activision announced its plan to acquire RedOctane in May 2006, and the transaction was completed on June 6, 2006. The deal was reported at roughly $100 million and gave Activision ownership of RedOctane and its Guitar Hero business. This is why Activision, rather than Harmonix, is the publisher normally identified with the franchise after its breakout period.
The corporate history is easy to mix up: Activision acquired RedOctane, while MTV Games acquired Harmonix later in 2006. Harmonix then created the competing Rock Band series. Activision subsequently moved Guitar Hero development to studios including Neversoft and Vicarious Visions.