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

"Jeong-Hui! Don't let it…get in your head! Take control!" Finn shouted, though had to heed his own advice as he clutched his hair.

It was his system, his instincts, his rationality, his experiences, the hairs on his skin, all of it told him the same thing: that creature can't be woken up. Reaching for the hood of the red-haired man, he was merely inches short, only grasping air as he watched Jeong-Hui stumble towards it in a trance.

'I get it now. The entire time…That ringing–that's why we were drawn here. Without even realizing it, this thing–it manipulated us, all for this moment,' he realized.

Even if he couldn't reach him, he could still stop him; that resolve was hardened in that moment, even as his brain ached.

'I have to stop him–no matter what. I'm sorry, Jeong-Hui, even if I have to kill you–I can't let you wake that thing up…!' He decided.

Assassin Versus Assassin