My companion is the strongest undead in another world is a Manga/Manhwa/Manhua in english language, Isekai series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.nl fastest updating comic site. The Summary is

Yuuma, a timid college student who has been summoned to a different world. It was thought that a fun life in a different world would begin with the cheating ability given to you, but what was waiting for him was cold-hearted and cruel treatment. His body is chopped up, fear and severe pain run throughout his whole body, and it is a living hell that never ends. “Why did this happen to me…” Aside from Yuuma’s heartbreaking cry, the blessing of “immortality” continues to regenerate his body. Yuuma was made into an experimental animal of researchers who were fascinated by the possibility, but he meets a pretty girl by chance in the facility — after meeting Eva, his life in a different world begins to change greatly!