System No. XX

System No. XX is the main antagonist and final boss of BlazBlue: Cross Tag Battle. It is a machine originally designed by Clavis Alucard to create new possibilities. However, a flaw in its design led to the machine invading the worlds it observed and using them as power bases. To prevent the machine from destroying worlds to create new ones, Clavis sealed the System away using the Keystones of the four universes (BlazBlue, Persona, Under Night In-Birth and RWBY).

However, the seal on System No. XX was accidentally undone by Clavis Alucard's daughter, Rachel Alucard, leading to the machine summoning warriors from the four worlds and having them fight in order to escape.