Gantz Omnibus 2

Gantz Omnibus 2 by Hiroya Oku. A Sci-Fi manga rated Mature, published in English by Dark Horse in 2019.

High schooler Kei Kurono and his childhood friend Masaru Kato are killed by a subway train, only to wake up in an apartment with a mysterious black sphere. The sphere gives them weapons and sends them out to hunt aliens in a brutal, violent game they cannot escape.