The New Hire is the Demon Lord

Dullahan, the demon lord who once terrorized the world, finally revives in modern-day Korea, eager to take revenge against the hero who defeated her centuries ago. But all she can find is his descendant, Yong-Rae Kim: a hapless, incompetent office worker with no prospects and no joy in life. Ending his life now would only be doing him a favor, so Dullahan decides to improve his life first… but Yong-Rae's two-timing girlfriend and his many strict superiors at work stand in the way. Can Dullahan crush the office hierarchy and awaken Yong-Rae's dormant hero abilities to make her revenge more satisfying? (Source: WEBTOON, edited)