Oof. This one’s tough.
For those not familiar, Btooom! is a mega-popular manga with a very similar premise, plot, and “feel” to Gantz. Shock value is the name of the game, including a stunning kill off in the recent material. Like Gantz, you never really feel like any outcome is assured, or any character safe from a horrific end. It was given an anime adaption not too long ago, though the anime kept things PG-13 rated. The manga most certainly did not.
There is a slight amount of actual fanservice in this manga, but almost all of the nudity exists only to make disturbing situations even more so.
Like many people, I was exposed to the anime before the manga. Madhouse will usually animate nipples when the manga calls for them, and so when their heavily censored (on broadcast) rape scene during the second episode turned out to be false positive censorship, I just assumed the manga did the same. Oh no. Not even close. Warning, many of the images in the following gallery are disturbing.
Btooom! manga (15 volumes, ongoing)
(The download link above contains several images not included in the gallery).
All things considered, this is pretty decent, if twisted, ongoing manga. I thought the anime was kind of boring, but the manga is anything but. This edition covers all twelve currently available volumes.