DelzGB

Within the virtual world of “Avalon Online”, Finn, or otherwise known as “Echo” within the game, was one of its greatest players. After logging off from a long session, however, he notices something awry outside of his apartment. 
The streets were flooded with chaos, screams echo through the night, and hell is loot loose. Monsters from Avalon Online are running through the streets, wrecking havoc. In the midst of this hellish awakening, Finn is greeted with the one gift bestowed to humanity in this dark hour:
[...Assessing life experience…] 
[Assessment Complete] 
[Name: Finn Thorne] 
[Age: 20] 
[Designated Class: Assassin] 
[Starting Level: 5] 
[A [Legacy System] has been bestowed to you for your accomplishments within Avalon.]
[Assimilation System Awakened.] 
In this twisted turn, as the world as it was once known is gone with the arrival of magic and monsters, the remnants of humanity must level up if they wish to survive. 
Finn is left in this dark, hopeless apocalypse, using his own experience from Avalon Online to best survive what seems to be an impossible task.

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Level-Up Apocalypse: Surviving With The Assimilation System

"Hey, erm…!" Gilbert stammered out.

Just as he glanced back to see what the ranger had to say, the air crackled. He looked up alongside Gilbert, witnessing the sky bend as a black object bled through space.

"What is that…?" Gilbert gulped.

—A book. The colossal, hard-cover book hovered at the world's summit, humming with an ominous tone. As its pages flipped open, it sounded like a plane flying overhead. It did not open to legible text, only presenting scribbled pages before a voice emitted from it–

[There are currently [67] humans that remain.]

"Sixty-seven…? No…What you said–you weren't lying…" Gilbert dropped down to his knees, shaking his head.

Though he was well aware of the dire state humanity was left in, it dug a needle deeper into his heart as Finn squeezed his fist. All he could see were the smiles of his friends, their laughs in those small, yet precious moments–the tiny reprieves amidst despair.

A Long Week Ahead