I Mistook the Demon Lord for My Master and I Am Invincible.

Lin Mo traveled through time and became a loser, and picked up a seriously injured old man as his master. Little did he know that this old man was the demon king who made the three worlds tremble. The elixir he made at random made the saints break through, the talisman he drew at random suppressed the ferocious beasts, and the sword he wielded split the sky - but he always thought that he was just an ordinary person.