Ghost Kraken is the third boss of Sonic Rush Adventure, appearing at the end of Coral Cave. Sonic, Tails, and Marine bust into a cavern at the end of the grotto, only to find the source of all those footprints they saw at the entrance: the robot pirate Captain Whisker, along with his parrots Mini and Mum, are stealing the Jeweled Scepter! To everyone's inter-dimensional surprise, Blaze the Cat bursts through the roof in a fireball at the same moment; Whisker directs the Ghost Kraken, presumably a creation of Eggman Nega, to fight the good guys while he attempts to escape.
Like all the Rush Adventure bosses, the Ghost Kraken's fight arena is 3D in contrast to the 2D platforming of the game's regular stages. This robotic octopus floats in the middle of a circular platform, extending its tentacles to attack in a variety of ways: direct strikes, sweeping attacks, flips to confine Sonic to a single section of the platform, and so on. He can be damaged by striking the green extremity at the end of just one of his tentacles (contrary to the rest of the Ghost series, who have a red control gem). When dealt enough damage, the Kraken drags the entire fight arena underwater and sprays ink everywhere, darkening the screen before attacking again. Different damage levels prompt the Kraken to raise or lower the arena from the top DS screen to the bottom, as well as changing his colour from green to red to blue.
Although the robot is defeated, it served its purpose for Whisker; the Captain has escaped with the Jewelled Scepter. Blaze explains that they are in her world, finally solving the mystery of why neither Sonic nor Tails recognised any landmarks after the storm that crashed the Tornado. They all head back to Southern Island take stock.