Raven Blade was the canceled Retro Studios role-playing game that left Metroid Prime as the studio’s only game in development.
Retro Studios was founded in Austin, Texas, to create several projects for Nintendo’s forthcoming GameCube. Among its early projects was Raven Blade, an ambitious fantasy RPG. The project was canceled during the studio’s turbulent early period, when Retro also abandoned or halted several other GameCube concepts.
Nintendo producer Shigeru Miyamoto visited Retro in 2000 and was dissatisfied with the state of its projects, but he saw potential in the studio’s technology. He suggested that Retro develop a new Metroid game after seeing its prototype first-person engine. The team then shifted its attention to Metroid Prime.
The other options are related to Retro’s early history but do not fit this clue. Project H.A.M.M.E.R. was a Nintendo project developed elsewhere, Dinosaur Planet became Star Fox Adventures at Rare, and Thornado was another canceled Retro concept. Raven Blade is specifically identified as the RPG whose cancellation helped leave Prime alone in production.