Demon's Souls is one of the best action role-playing games of all time. It was also the first dark fantasy game created by Hidetaka Miyazaki, hence ushering the golden age of FromSoftware with the release of the Souls games.
Demon's Souls laid the foundation of Miyazaki's creative genius. The game contained the essential blueprint of what makes a successful Souls game - integrity, consistency, and quality. Similar to Dark Souls, though Demon's Souls is characterized for its perceived difficulty, it contains some of the best level design, boss fights, music, and atmosphere among all the Souls games. Though it is the first, it is among the best in the series.
Together with Dark Souls, Demon's Souls is one of the best and most influential games of all time.