Before you start the game, you need to know the game background. It is the story behind the game:

Convinced that serving God only imprisoned and robbed him of his free will, Baalon broke loose from the heavens to claim his independence from his creator. Believing that human beings were nothing more than inherently inferior creatures controlled by their sinful desires, Baalon sought revenge by plotting to enslave all of humanity, God's most precious creation.
In order to execute his sinister task, Baalon created the Gate Stone which granted him the ability to enter different worlds, leaving the human world defenseless and vulnerable. Immediately, Baalon set his sights on wreaking havoc and destruction on the inhabitants there, enveloping the Esior Continent into unrelenting tragedy.
The realm of Hades, unable to care for the growing swarm of deceased spirits clamoring for eternal rest, was desperate to end this bitter massacre. In order to restore order back to the human world, they sent a mercenary troop comprised of their best warriors called Zero Force to wage war against the Sinistry of Devils of Demon King Baalon. Although they fought valiantly, Zero Force was lamentably defeated and Esior fell into the hands of the Sinistry of Devils.
With no one left to oppose them, the Sinistry of Devils easily infiltrated and seized control of the human world. Although the number of the Sinistry of Devils was few, they cruelly exercised their supremacy deep within the cities of countries, transforming treacherous traitors into their loyal subjects and brave generals to insignificant beggars. Gripped by the wickedness of the Sinistry of Devils and plagued by the recent attacks of monstrous creatures, humanity found no relief in sight and suffered in endless turmoil.
Despite enduring infinite pain and sorrow, human beings fiercely hoped for the arrival of a hero to drive out their foreign invaders and bring back peace. Fortunately, a famous prophet named Jexius received a vision from God and delivered this hopeful message:
According to the prophecy, anyone who wishes to be saved from this torturous hell must await the arrival of the Holy King who will purify the putrid blood of the Sinistry of Devils. That will be the WonderKing, who will save the world from the darkness.

Knowing about the story, then you can start your journey in the game. Have fan!