The Boss Gate Key, or just Boss Key, is a recurring object in the Sonic the Hedgehog series. It is a special type of key that is needed to unlock Boss Gates, which grant access to the local levels' bosses.
The first Boss Keys appeared on the PS3/Xbox 360 version of Sonic Unleashed under the name Gaia Keys (ガイア神殿の鍵,Gaia shinden no kagi?, lit. "Key to the Gaia temple"). In this game, they are needed to unlock certain Gaia Gates (called "Boss Gates" by Professor Pickle). Story-wise, they each grant access to their local Gaia Temple via their Boss Gates..
While there are seven Temples of Gaia in the world, only four Gaia Keys are available in the game. This is because the player automatically accesses the Gaia Temples in Eggmanland, Mazuri and Chun-nan without a need for a Gaia Key.
Each Gaia Key is split into to halves: one Sun Key and one Moon Key. Both these keys are needed to form one whole Gaia Key. To obtain both halves for a key, the player has to clear the main daytime Act (which holds the Sun Key) and nighttime Act (which holds the Moon Key) on one continent. This means, for example, to get the Gaia Key for Spagonia's Boss Gate, the player must clear both the daytime and nighttime version of Rooftop Run Act 1. When playing through these Acts for the first time, the Gaia Key halves will serve as the Goals; after that, they will be replaced with Goal Rings.
It should be noted that different continents' key halves cannot be used to substitute others. This means, for example, that Holoska's Sun Key cannot be used as a substitute for Spagonia's Sun Key if the player wants to form a whole Gaia Key.
In the console/PC version of Sonic Generations, the Boss Keys are needed to open the Boss Gates in the White Space. Each Boss Gate found in the game requires three Boss Gate Keys to be unlocked, making a total of nine Boss Gate Keys to be found.
After both Classic Sonic and Modern Sonic clear all the Acts in an Area of the White Space, three Boss Gate Keys will appear around that Area. However, these keys will be surrounded by unpassable shields that makes them unobtainable. In order to obtain them, the player has to clear one Stage's Challenge Act in either Act 1 or Act 2 for each of the keys. Once a Challenge Act has been cleared, the shield will be lowered and the Boss Gate Key can be picked up.
In this media, the Gaia Keys open up for the inner chambers of the Gaia Temples. In turn, they have been entrusted to select families who keep their duty a secret. Hoping to resolve the Shattered World Crisis, the sage Gregorios revealed the existence of the Gaia Keys to the Freedom Fighters and provided them with the identities and locations of the keyholders.
↑Sonic Team (9 December 2008). Sonic Unleashed. PlayStation 3. Sega. Area/Level: Collection Room. "Prof. Pickle: Yes, to operate the Boss Gates, you'll need to assemble the Boss Key. The daytime and night stages should each hold a separate key. Those two can then be fused into a single key, and used at the gate."