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

The moment they both shared in their relief, all of it was washed away by that "sensation." A ripple through the stagnant air; vibrations humming quietly, though felt like a malignant omen against his skin.

'What…is this feeling?' He questioned.

As sweat dripped down his skin, he slowly turned to where the enigmatic entity was left suspended. It was no longer frozen; its eerie, black body moved as its arms pulsated, coiling with unfound strength.

[Restricted existence recognized.]

–It surpassed logic, witnessing such a level be assigned to the mysterious lifeform as its serpentine lower half slithered across the tile. The presence it invoked was suffocating, as if the air became solid.

Two Days Later