Joushou Maou no Yarinaoshi

It has been 500 years since Sieg, who was said to be the strongest demon lord has been taken down by the legendary heroine. The adventurers guild is fully centered around and devoted to the hero's descendants, just because they are blood related to the hero. A low level adventurer called Aru, however, got deceived by one such hero, and ended up picking up a cursed blade for the sake of his female companion Yuuna. However said cursed blade inhabited the full power of the demon lord Sieg. Having been picked as the reincarnation for Sieg, and now having the greatest power at his finger tips, the former subordinate Aru went on a journey with a beautiful Harem to destroy the depraved hero.
A story of a unrivaled demon lord based on a web novel.

(Source: HJ Bunko, translated)