Forced Revenge

From alpha to omega, from a soldier galloping on the battlefield to a
canary kept at home, Major General Augus of the Union was captured as a
war prisoner by the enemy Empire. Under torture, Augus was forced to
undergo an illegal s*x change turning him into an omega. Once an Alpha,
Augus struggles after being forced to endure an omega's estrus. The only
one he can rely on in such a time is the Empire's General, Ed.
Ed thought the third prince of the Empire was going to do something
stupid in his life, but he didn't realize the prince would force his rival to
become an omega. Standing over his once strong, independent rival, Ed's
alpha pheromones kick in and demand that he mark this once proud man as
his.

Source: Exiled Rebels Scanlations