Black Phantoms can crap all over your gaming experience.
The Inevitable Player Vs. Player
Black Phantoms are the bane of Demon's Souls co-op. These jerks will ruin your day. One of the drawbacks to playing in Body Form is that your game is now vulnerable to Black Phantom invasions. These are other players who will enter your game and their only goal is to kill you and your party. Black Phantoms can earn massive soul rewards by killing Blue Phantoms and they will regain their own Body Form if they defeat the Host player. In-game enemies will totally ignore them.
The good news is that a Black Phantom will always be a lower level than the host player. You can only be invaded by one Black Phantom at a time. Since Black Phantoms invade at random, you may play for hours without seeing one. The bad news is that there is no way to turn this "feature" off. You may think that this isn't a problem, because you can simply summon two Blue Phantoms while in Body Form, and you'll have a three-on-one advantage.
Wrong. The god-awful news is that there are people who have been playing the game since 2009. You'll recognize them right away. They invade without wearing any armor. They'll ninja roll all over the place and then stab you in the back like a, well, like a ninja. There's also a fairly well known glitch that allows even low level characters access to very powerful weapons. Eventually you'll get good at PvP and this will present less of a problem. But no matter how good you get, it's still easy to die in the game, whether you're in Body Form, Soul Form, or a Blue or Black Phantom.
I should note that you can play the whole game in Soul Form. You'll only have half of you health, but you will never be invaded. If you are online you'll experience the passive co-op features (white ghosts, bloodstains, and messages) but you won't be able to summon other players.
The other stuff...
Demon's Souls has an incredible amount of extra features, including intelligent level designs, game changing items, and a ton of replay value. A haunting soundtrack punctuates the boss battles. There is an elaborate weapon creation system which uses materials that aren't even guaranteed to show up on every playthrough. There are both World and Character Tendencies, where your actions can change the strength of enemies, open up new areas, and even spawn powerful NPC Black Phantoms. To get a Platinum Trophy you need to finish the game multiple times, and each time you start a new-game-plus the enemies naturally become stronger. If you are having a hard time with the game, but hate to admit defeat, I strongly suggest glancing at the Demon's Souls Wiki. Ease into it on your first playthrough, or you'll ruin the game for yourself. If you want to see my first-hand account of one of my early co-op experiences with the game you can check it out here.
According to the Co-Optimus definition, Demon's Souls is not that great of a co-op game. In fact it's not a co-op game at all. The only way you can play with someone else is to send out an open invitation for a random jerk to invade and ruin your game. We usually don't cover versus or PvP games here, but Demon's Souls is a kind of hybrid exception to the rule. You can play the game without encountering an enemy player, but it takes a little luck.
What Demon's Souls has going for it is that it is an incredibly rewarding experience that uses cooperative gameplay elements. If you get a good party together you will have few problems, even from Black Phantoms. The randomness of the game makes this very difficult. If you enjoy a little PvP with your co-op, I cannot recommend this game enough. Demon's Souls is one of the most incredible games from this console generation. Right now you can find it for under $20 bucks, and I highly recommend it, especially since you won't be able to play online after October. We have our fingers crossed that the upcoming sequel, Dark Souls, will at least have an option for a more co-op friendly experience.
Verdict
Co-Op Score
Overall
The Co-Op Experience: Players can summon or be summoned into each other's worlds. The goal of the host is to defeat the boss to progress the game. Summoned players need to conquer the area boss and reclaim their bodily form, allowing them to summon other players to their own world so that they may progress in turn.
Co-Optimus game reviews focus on the cooperative experience of a game, our final score graphic represents this experience along with an average score for the game overall. For an explanation of our scores please check our Review Score Explanation Guide.