'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.
Individuals that pass through a Shadow Sphere, using it as a Portal are corrupted, either through the influence of the sphere itself, or more likely through the corruption caused by The Hunger and the Shadow Entities. The corruption induced causes physiological changes, such as turning the eyes of the individual black, as well as giving them significantly increased strength. Additional psychological changes are also induced, causing reckless, mindless and animalistic behavior.
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.