I Become The Youngest Disciple of The Martial Arts Leader 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 in the martial world, leader of the Hojeongdan, Lee Hwan! During a mission, he battles the Heavenly Demon of the Demonic Sect, Deung Gwang-cheon, and loses consciousness… When he opens his eyes, he’s at home? No… This is my childhood home? It’s hard to believe, but if I really have returned to the past, this time I won’t just stand by and do nothing! On the night of August 8th, when he was 17 years old, a sudden, splitting headache strikes, and something comes to mind. “Cheonsu Ilgi Hap (Thousand Hands One Energy Unification), Deukwon Gang (Obtain Original Strength)…” He muttered absentmindedly and then paused. “Wait, this is the Absolute Demonic Art of the Heavenly Demon! How do I know this technique? Am I going crazy?!”