https://mangaplus.shueisha.co.jp/titles/100040
From the Fandom Wiki-
Gabimaru the Hollow, a former ninja assassin of Iwagakure who is known and feared as a heartless husk of a man, spends his days on death row wondering when an executioner skilled enough to so much as harm him will arrive, as he thinks nothing of seeing an end to his meaningless existence... Or so he thought. After a skilled female executioner belonging to the Yamada clan by the name of Yamada Asaemon Sagiri confronts him, she makes the young ninja open his eyes to the fact that he wants live in order to reunite with his wife and offers him a proposition: if he ever wishes to see his beloved wife again, he is, under the auspices of the shogunate, to embark on a perilous voyage to a mysterious mystic island said to house the Elixir of Life. Should he be the one among many rival death-row fiends and scoundrels to find the elixir and return with it for the shogun, he will earn a full exoneration, and, more importantly, a chance at an ordinary married life with the light of his life--the woman who made the world seem not so ugly. Gabimaru gladly accepts the task but soon finds out that he has recieved more than what he bargained for. Is what awaiting him and the other contenders Hell? Paradise? Or Hell's Paradise?
