First Referenced:
'I was told that everyone I'd served with in the military died shortly after I left. (Part 3)'
The Shadow Sphere is a 10 foot sphere, existing at the center of The Ink on Earth 44. Its exact nature is largely unknown, other than its abilities to generate The Ink and connect in a fashion similar to a Portal to various other extreme and dangerous realities, such as that inhabited by the Preacher. It appears to have the ability to corrupt individuals who pass through it as a portal, which turns their eyes black, gives them significantly increased strength and makes them particularly dangerous.
Whether the Shadow Sphere is a malicious entity in its own right, or simply a construct is currently unknown. The Sword knows of its existence, as does Cristina Thompson, who used her mental control of him to attempt to deploy troops through it when it connected to the reality that the Preacher had escaped to, which was filled with blood and mountains of corpses.
The rebellion on Earth 44 resulted in the retreat of The Sword and many of his troops through the Shadow Sphere, which turned them into the Black-eyed Army