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

Something about it seemed off as he stared for a moment, finding the spellblade just a meter or two in front of the organic shield.

"Charlotte, watch out!" Finn shouted.

The limb of the moon-residing entity shifted again, this time changing from that of a triceratops' skull to the roaring maw of a tyrannosaurus. With clutch haste, the young woman moved herself out of the way, though it didn't stop the nebulous entity from continuing to morph and spread itself out.

"What the hell is it doing now–?!" Finn questioned, catching up beside the spellblade.

"I…I don't know!" Charlotte shouted back.

A buzz transmitted through his helmet, seeming to be shared with his companion as they shared a glance with one another–

["//Finn? Charlotte? Are you hearing this–? It's—Nikko!//"]

Finn held the side of his mask, "Yeah, we're here!"

Earth's End