Haenguneul Bireoyo, Yongsanim!

After a great hero sacrifices himself to defeat the nefarious Demon Lord, an angel grants his wish to be reborn with his memories intact. Now known as Hero Baek, he puts his memories to use and writes a web novel depicting the story of his past life. In time, he lands a job at an entertainment company, working for the genius and intimidating CEO, Yeon-Hwa Do. Unbeknownst to Hero, his new boss is none other than the reincarnation of the Demon Lord. When his web novel catches her attention, the two mortal enemies appear destined to collide once again.

(Source: WEBTOONS)