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

"—" Finn didn't have a response.

"If you're so curious, I caught you from slipping through realities again," the Evanescent admitted, making a dropping motion with his hand. "With the fragility of the universe at this moment, it's easy to do."

"…Thanks?" Finn said hesitantly before stepping forward. "Listen, you know more than you're letting on, don't you? All of this…"

The Evanescent glanced at him with a hollow look permeated in his dull irises, "A history lesson won't change anything right now. You'll learn everything, should you survive. I'm sure of that."

"Hold on—who the hell are you, really?!" Finn questioned, attempting to approach.

Just like that, the elusive figure was gone with the wind. Again, he was left with more questions than answers, far removed from his last memory.

'The fabric of reality…Is that why I blanked out earlier?' He considered.

A Quiet, Empty Universe