Infinite Breakthrough

Nao Tsurugi is a hardworking salesman pushing himself to the brink to care for his sick family. Because of his honest nature, his sales performance remains poor, leading to a tragic end when an irate client stabs him to death. He awakens in a mysterious amusement park where only those who have died are gathered.
A message informs him that completing quests will return him to the living world, but the clear condition requires defeating boss monsters capable of instantly killing a human. To make matters worse, Nao receives a bottom tier star 1 unique weapon. Just as despair sets in, he discovers that his weapon alone possesses the power to grow infinitely.