The Stone of Sealing is the creator of the Undead and the overarching antagonist of Kamen Rider Blade.
It is the entity responsible for the Battle Fight, as it created the Undead to determine the dominant species on Earth by having them fight each other. It is also responsible for spawning Darkroaches in the event the Joker Undead secures victory.
10,000 years before the present day, the Stone of Sealing created the 53 Undead and had them duel each other in a battle royale known as the Battle Fight to determine which species would become the dominant race on Earth. The Human Undead proved the victor of the Battle Fight and so humans became the dominant species ever since, with the rest of the Undead being sealed within the Sealing Stone.
However, in 2004, a second Battle Fight was triggered by Hiroshi Tennoji after he excavated the Sealing Stone and unsealed the Undead from it, all as part of a scheme to become ruler of the world by rigging the Battle Fight and having his own personal artificial Undead Kerberos be the victor.
Though Kerberos was destroyed and Tennoji's plan was foiled, the defeat of Kanai left only one Undead remaining, the Black Joker, representing Death, leading the Stone of Sealing to determine Joker as the winner and begin spawning massive amounts of Darkroaches to destroy all life on Earth so life could start anew.
However, Kazuma Kenzaki managed to subvert the destruction of humanity by becoming a Joker Undead himself, leading the Battle Fight to be prolonged indefinitely.