occult ball

オカルトボール

Special items appearing as the focus of Urban Legend in Limbo. They look like dark-purple orbs made of some reflective glass-like material.
They are actually "Power Stones" coming from the various power-spots of the world, and were used by Sumireko Usami to tear a hole in the Hakurei Barrier and enter in Gensokyo. They are also the cause for the appearance of the urban legends in Gensokyo.
Apparently, one of them comes not from the Outside World, but from the Lunar Capital, making it an extremely dangerous object to have and use.

In Legacy of Lunatic Kingdom, it is revealed Sagume Kishin was the one who responsible for creation of these orbs.