Demon's Souls has spent a year or so developing a fanatical cult following in the US and Japan, and while not everyone will understand why, those who do are in for a treat. This is not a game that inspires lukewarm feelings. If it grabs you, it becomes an obsession, dragging you back for hour after hour of wandering and fighting in its grim and gloomy fantasy world. If it doesn't, you'll hate it with a vengeance.

The reason why is simple: where other video games are bending over backwards to please, focused on giving the average player a mild challenge and an entertaining ride, Demon's Souls just doesn't care whether you like it or not. It is relentlessly difficult. It forces you to fight and die and return and fight and die again and return and fight and die again. It can feel like a pointless struggle. Its difficulty curve isn't so much a slope as a brick wall with carefully placed handholds that might just enable you to climb up if only you summon up the stamina required. Let's just say that if you play games for easy, stress-free entertainment, then Demon's Souls is not for you.

In fact, the first hour of play might leave you suspecting that it's not for anyone. As a lone hero, entering a desolate fantasy kingdom in search of fortune and glory, you'll find yourself annihilated within the first 10 minutes in a virtually unwinnable boss battle, at which point you'll find yourself in spectral form in the mysterious chamber of "the nexus". You can have your body back, you discover, but only by returning to the fray and slaying a major demon. The catch? Until you do so, you'll be a weakling with a pitiful health bar, clad only in the most basic arms and armour, and easy meat for the countless zombie soldiers, demonic knights, gelatinous creatures and assorted beasties that stand in your way.

Undaunted, you touch an archstone that transports you to a bridge leading to a lonely castle, watched over by vast, menacing dragons and patrolled by a host of moaning, undead gits. You'll make your way through the first few without any problems, slashing them to bits and harvesting their souls. If you're lucky, you might even make it through the lower battlements, up the tower and to the top on your first attempt. Don't pat yourself on the back quite yet. One quick encounter with some double-hard bastard and you're gone.

Forget checkpoints. You respawn at the start point and - to your horror - realise that everyone you've slain has also returned, and that, until you get back to the point of your unfortunate departure, all the souls you've harvested are forfeit too. So you grind on through, and die again. Then grind on through, and die again. You'll get a little bit further each time. Your skills with sword and shield improve. You'll find new weapons and new caches of health and special items, and you'll work out new strategies against your foes. All the same, Demon's Souls keeps grinding your face in the dust. The game's staunchest defenders will tell you that this is part of the appeal. We're not 100 per cent convinced. While we think Demon's Souls hits the balance between tough and fair a lot of the time, there are long periods where you'll wish you were playing something easier, or a little more friendly. Does Demon's Souls really have to be this grim?

If you can, hang in there. Eventually you will win through. You'll defeat your first boss demon, get your hands on new goodies, and begin the work of piecing together the minimalist plot. All the same, it won't be long before you'll die again, and so the whole process repeats.

Were this any other game we'd be telling you by now to give it a wide berth, but the fact is that Demon's Souls has something that keeps you coming back for more of the same old punishment. Partly, that something is atmosphere. Demon's Souls has a fairly limited palette of colours and environments - gloomy tunnels, stone walls, abandoned courtyards and poorly-lit chambers are a real speciality - and yet the game has one of the most immersive, tangible fantasy environments of any fantasy game we've played.

The lack of other characters plays a key part in it. As with the magnificent ICO or Shadow of the Colossus, there's a real feeling that you're isolated in a world with its own history and its own twisted logic, where you won't really understand what's going on until it's almost too late. It's creepy and, thanks to the game's brutal approach to failure, horribly tense. In a game where dying matters, every unblocked blow and trap sprung matters too.

It also helps that the basic game mechanics are very sound. The combat is relatively simple, with blocks and attacks played out using the four shoulder buttons on the Dual-Shock 3 controller, but timing is everything, and the game makes you think hard about which weapon should be used in which situation, and about how you tackle enemies in groups. It might seem odd, but there's something grimly realistic about Demon's Souls' approach to fighting. You don't feel like an unbeatable hero, but a tough adventurer who might - just might - be able to scrape through the next big fight.

The game's RPG features, meanwhile, are both elegant and restrained. You can choose a class at the start of the game, but the way the game's levelling and equipment system works ensures that there's nothing to stop a magician bearing arms or a warrior wielding spells if you choose to take them in that direction. A bare minimum of controls and screens is involved to handle spells and inventory, and even the levelling and equipment systems lock into the game's grim kill-or-be-killed vibe. Weapons and armour get damaged as you use them, meaning you have to take care of them or they won't take care of you.

Meanwhile, you can only level up back at the Nexus by swapping souls for levels, which means getting back to the archstone in one piece after bagging as many as you can. This constantly leaves you with a dilemma: do you return and bank the souls you're carrying now, knowing that you'll have to repeat your progress on the current level once again, or risk going a bit further but losing what you've got at the next big battle? It's a scary choice, and only you can make it.

Finally, it's hard to imagine a more intelligent way of integrating online play. Demon's Souls is very much a single-player experience, but sign in to your PSN account and, as you traverse the world you'll find notes from fellow players inscribed on the ground, or bloodstains that reveal a fatal moment. With experience, you too can leave these markers, giving the game a community feel without compromising its sense of lonely struggle.

Come up against a section you just can't beat, and you can also summon in the ghosts of other players to give you assistance for a while. If they help you, they can regain their bodies, and the same goes for you if you're summoned to another player's game. Of course, the online mode also has its darker side. The ghosts of players of a similar level can force their way into your game and attempt to hunt you down - another good reason to get your body back a.s.a.p.