The Shadow God is one of the Seven Gods. His domains are peace, death, solace, and mysteries.
Relic
His known relic is the Shadow Lantern — a small black stone lantern engraved like serpent scales, ranked Divine tier I. Enchantment [Gates of Shadow]: devours light and can contain then release an infinite amount of shadows. Sunny obtains one (his Divine Memory) at chapter 654.
Death Aspect
The Death aspect of the Shadow God is embodied in the Soul Serpent's [Slaying Blade] ability, which ignores the will of greater beings. It is one of the rare abilities in the story that can wound daemons.
Link to Sunny
Sunny's Shadow Slave Aspect appears to point directly to the Shadow God. The exact nature of the link — patron, parent, source — remains a central narrative mystery, not fully resolved by Vol 11.