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

Above him, the hand-shaped entity morphed again, its flesh twisting and condensing, being shaped into something else.

A feminine shout filled his ears before he witnessed a flash of visceral fire cut across the shifting form of the Moon. Recognizing that cutting swiftness was easy for him, even more so as he saw the figure land beside him with a graceful slide.

Though they were equipped in the same, black, form-fitting armor that showed her feminine physique, he already knew–

"...I made it! I'm sorry I was late–!" Charlotte quickly apologized as her helmet dismantled briefly, showing the genuine guilt in her pearly irises.

Finn disregarded any need for an apology, stepping in front of his companion, "Save it! We've got bigger problems right now!"

The Terror of Gravity