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

Recalling the encounter with that serpentine-bodied life form, it made the hairs on his body stand. Everything about it was "unnatural", like witnessing the supernatural firsthand.

"The only objective here is to survive, right?" Jeong-Hui reminded him. "We don't need to overstep into anything that might lead to an early grave."

"You're right. Whatever this all is…Don't want any part of it," he agreed, moving along.

Hiking along for some time brought the two assassins to a new region of the peculiar world—

A colossal stretch of land made up of giant, colorful mushrooms that almost seemed to play the role of trees.

"Careful," Finn advised, looking up at one of the oversized mushrooms that loomed over him.

"Mhm," Jeong-Hui quietly mumbled, carefully moving forward.

Fungi