Demon Souls was suppose to be free for a limited time according to the description but it has been free for quite a long time now which is quite odd. So anyway, the game is a free to play (I guess forever) RPG with demons involved as the name says. You basically get to capture, evolve and battle with demons besides using your character for battle. In a way, it's kinda similar to the Megami Tensei game series. As for the story, pretty basic and not that appealing along with the characters (well at least so far I played).
Other than that, I can't really say much because it's just a typical RPG you probably see everyday but however it's decently fun.
Conclusion: A decent RPG game.
Note: Available for free on the US and Japanese app stores.