Mortal Kombat Warehouse

A refugee orphaned by Shao Kahn's conquest, Rain grew up under the protection of the Edenian Resistance. An exceptional fighter, he soon rose through their ranks. As his reputation grew, so did his arrogance. When Rain demanded leadership of the Resistance forces, he was refused. Infuriated, he turned against his rebel comrades and betrayed them to their sworn enemy. In payment for this treason, Rain was promised his own army by the emperor Shao Kahn. Power would be his - no matter the cost...

Shao Kahn had used Rain to crush the Edenian Resistance but had not granted him an army.
For this betrayal, Rain drowned the emperor in his own blood.
A grateful Raiden thanked Rain for eliminating the emperor and saving Earthrealm.
His heroics were befitting of a son of Argus.
Rain's lost heritage was a revelation to him. That he was a direct descendant of an Edenian god proved his superiority.
Power was his by right.
His divinity confirmed, Rain's path was clear: He would use Shao Kahn's army to rule not just Outworld, but all the realms.
To Raiden's surprise and horror, Rain's first target is Earthrealm.