The "Quake" series continued with "Quake II" in 1997. Activision purchased a 49% stake in id Software, making it a second party which took publishing duties until 2009. However, the game is not a storyline sequel, and instead focuses on an assault on an alien planet, Stroggos, in retaliation for Strogg attacks on Earth. Most of the subsequent entries in the "Quake" franchise follow this storyline. "Quake III Arena" (1999), the next title in the series, has minimal plot, but centers around the "Arena Eternal", a gladiatorial setting created by an alien race known as the Vadrigar and populated by combatants plucked from various points in time and space. Among these combatants are some characters either drawn from or based on those in "Doom" ("Doomguy"), "Quake" (Ranger, Wrack), and "Quake II" (Bitterman, Tank Jr., Grunt, Stripe). "Quake IV" (2005) picks up where "Quake II" left off – finishing the war between the humans and Strogg. The spin-off "" acts as a prequel to "Quake II", when the Strogg first invade Earth. "Quake IV" and "Enemy Territory: Quake Wars" were made by outside developers and not id.