Archaeologist Dr. Ethan Harrow unearths a vial containing the ancient substance known as Sumerian Sapphire. When the government tests it on Sergeant John Reaper, he undergoes a terrifying transformation into a monstrous Grim Reaper, wreaking havoc in a secure military lab. Only Captain Alex Mercer and Dr. Lily Sterling survive, using a counteragent to momentarily restore Reaper's humanity before his death.

However, the threat isn't over. A new, shape-shifting Reaper, Reaper Prime, infiltrates the survivors' ranks. Mercer and Sterling, alongside a new Special Ops team, Ghost Strike, uncover its true nature and ultimately destroy it with a refined Elixir of Dawn.

Finally, a new strain of the Sumerian Sapphire spills, consuming everything it touches. As chaos ensues, an elite team of alien angel soldiers, the Celestial Guard, arrives to combat the menace. Led by Seraphim Orion, they join forces with Mercer and Sterling to destroy the liquid's core, ending the threat once and for all. With the knowledge of the Sumerian Sapphire's origins in their hands, Mercer and Sterling vow to prevent future catastrophes.