"Qualification"
The Old World known as [Sick Focus] has had a strict limit on the number of "Plague Masters" since the birth of the World. It's not that the World wants to limit such strong entities, but rather the World can only endure and accommodate so many Plague Masters.
However, the Sick Focus World is quite special, unlike other worlds which were 'rooted' between planes at birth and maintain fixed coordinates. Like trees in a forest, their roots deepen with time without leaving their position.
Yet the Sick Focus is a 'living world', with a kind of special mutation forming the sky-reaching tentacles— "World's Foot"—allowing this world to crawl forward in the relatively loose Outer Plane, even 'parasitize' and 'infect' other worlds.
It is exactly this world trait that swiftly drew the attention of the Main Material Plane.
