I Died 999999 Times – Infinite Level Up with Invulnerability Skills is a Manga/Manhwa/Manhua in english language, Action series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

The strongest hunter guild, “Byakuren”, is praised as the world’s heroes after defeating the Demon King. However, the truth hidden behind their power is utterly inhumane. Their strength comes from a cruel secret—they imprison two siblings, Makoto and Minori, who are descendants of the undead, repeatedly killing them to forcibly farm experience points. To survive—and to take revenge—the siblings make a mad choice: killing each other to level up. “Let’s begin—our revenge.” Awakened through their deadly struggle, the two carve a path of vengeance.