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

Charlotte worriedly asked, "What is it?"

There was no need for an answer as the grating sound met their ears even from miles away. Finn witnessed it from afar: the "ceiling" of the city parted like an automated dome. From it, what looked like pitch-black silhouettes rose out, moving together in a unit.

"What are those?" Finn wondered. "Can you see?"

Jasper answered to the best of his ability, "They look human…human shaped, at least? Like mechanical, sorta–I'm at a loss here."

"Wait, like gundam?" Finn asked.

"Now that you say it–yeah, like that. Wait, they're coming this way, I think," Jasper pointed out.

"They are!" Charlotte confirmed.

How Outsiders Are Greeted