There more than one boss in the game (secret boss). There a few things each of the characters have to achieve before you unlock this new extended story mode & boss. There was a KEY KEEPER that eventually turned evil & too powerful, that was killed by the animals in Foenum. This evil Key Keeper spirit was banished to another dimension. The animals of Foenum made sure this evil Key Keeper spirit was not sent to the dimension where the Predators where banished to.
The evil key keeper was able to free it self from its dimensional prison from using the powers of the predators, the devourer, the animal of Foenum, the new key keeper & the enchanted key.
With this new power the evil key keeper resurrected it self with new bodies (Orochimaru, Pain, Nagato) , abilities , magic (white magic & black magic = grey area/grey magic) & transformations (SkullGirls Double fused with Bloody Marie) . The Evil key keeper is a threat to Foenum, the predators and all the other dimensions.
The evil key keeper can take many forms (male, female, non gender characters )(Steven Universe fusion).
You wouldn't know what type of character it will start of as until the announcer shouts "fight/battle/stamped" & the fight begins.
There would be evil key keeper transformations & powers are random in each battle (minimum random transformations would be 7 or 8 depending what other playable characters are added to the roster).
The evil key keeper could transform into living or dead friends, foe, rivals, characters they &/or their community looked up to, characters they read about, character they feared, family members of each of the characters to make each story mode boss fight unique for each playable character