Since its release on PC and Xbox 360 this time last year, The Elder Scrolls IV: Oblivion has racked up awards and perfect scores aplenty. Set in the realm of Tamriel at the end of the Third Era, gamers were instantly wowed by a vast landscape, amazing graphics and a gripping plot. After the main game there quickly came a succession of downloadable content which included the infamous Horse Armour and the critically acclaimed Knights of the Nine side quest. Now, a year later, Bethesda is unleashing its first expansion, The Shivering Isles.

To be released simultaneously on 360 and PC, The Shivering Isles focuses on another Daedric Prince, Sheogorath, the slightly-bonkers (and then some) Lord of Madness, who will be familiar to fans of Morrowind as He Of The Fake Apocalypse and Rain of Dogs.

Sitting down to play, it might seem a misnomer to say that The Shivering Isles is beautiful, particularly when considering the lord of this strange realm. When a strange portal appears in Niben Bay and insane people start wandering out, it's up to a hero (yes, once again, that mantle has fallen to you, the player) to go in and see what the Mad Lord wants. But when was anything that simple?

The portal itself is a striking thing and completely different from other Oblivion Gates. Made up of three faces which symbolise Sheogorath's multiple personalities, the swirling vortex at its centre is not as terrifying as those of Mehrunes Dagon but that doesn't mean it's safe... far from it. Sheogorath is a picky bugger when it comes to his Champion and so demands a little worthiness in the only way known by Daedric Princes: a little slaughter - one dead Khadjit coming right up! The poor insane creature just happened to be in the vicinity but oh-the-irony! Sheogorath really doesn't like cats.

But the vetting isn't quite over yet. The Mad God's indentured servant Haskill is waiting for you on the other side. Before progressing further, Haskill wants a civilised chat, so civilised in fact that you have to sit in the offered chair before he'll talk to you. But such niceties are not in vain and this is where usage of the word 'beautiful' comes in. Rather than stepping through a door, the walls transform into a flock of multi-coloured butterflies (over four thousand of them - and one moth - according to Bethesda). After picking up your jaw, it's time to go find Sheogorath.

The look is unlike that already seen in the game

The striking thing about The Shivering Isles is that it's totally unlike Mehrune's realm. It's easy to forget that the red-soaked skies are not the norm and that each plane of Oblivion is shaped by the Daedric Lord who rules it. Despite its name, which summons up images of arctic vistas and falling snow, the Shivering Isles is actually an autumnal realm. The blue-gold sky is positively pleasant and the landscape is dotted with large crystalline structures, huge mushroom trees and long vine-like roots; but the land itself is split in two, each half filled with strange creatures, mist, moss and swamps.

The Fringe, the area of the Shivering Isles in which you arrive, is the gateway to the Mad God's realm and what would a gate be without someone to guard it? Exploring Passwall village and chatting to helpful (and strangely sane) locals leads to the eventual encounter with the Gatekeeper - a tattooed giant with a Hannibal Lector-like mask and homicidal tendencies who dispatches Champions for breakfast. But what's a 20ft giant to the Hero of Kvatch - the bigger they are, the more satisfying it is when they fall. From the Gatekeeper come two keys which unlock the two entrances to the Isle proper. One road leads to the north side of the city of New Sheoth, known as Bliss, the other to the south leads to Crucible. The final portion of the city is dominated by Sheogorath's Palace and its striking sky filled with pink and violet stars. It's here where the Mad God resides along with Thadon, the Duke of Mania, and Syl, the Duchess of Dementia. Each rules a portion of his city although Thadon is more interested in feasting on intoxicating and mind-altering food, and Syl is just trying to avoid being assassinated.

20-30 hours of new questing can't be bad

Even by The Elder Scrolls' standard, Sheogorath is one weird character. His blonde locks, beard and cane remain from Morrowind, as does his unique sense of humour as he entreats the player to come back soon lest he'll strangle them with their own entrails. But the Mad God isn't all bad, bestowing the ability to summon his manservant Haskill, which is worth it just to see Sheogorath's amusement and Haskill's stoic ambivalence. Indeed, Sheogorath has to be one of the most filled out inhabitants of Oblivion and his rapid transition between fury and hilarity is terrifying and amusing at the same time. He has some great one liners too: "Cheese for Everyone!" being my current favourite.

Between threats and amusement, the Lord of Madness explains more of his reasons for wanting a Champion to save his realm. With the death of Tiber Septim, the Third Era is ending and the Greymarch, headed by Daedric Prince of Order, Jyggalag, is coming to wipe the Daedric realms clean. Not a good thing and Sheogorath is powerless to stop them - which is where you come in and so the Mad God sends you out into his world.

The gameplay and controls remain unchanged but it's the quests we're really interested in. The early quests involve exploring the Shivering Isles, torturing the locals with lightning bolts and restoring Xedilian by finding the Resonator of Attenuation. This little gadget comes into play later on in the expansion in a suitably twisted manner. It draws people towards Xedilian dungeons - another of Sheogorath's amusing diversions which confronts all those who try and enter the city - complete with sick games right out of the movie Saw. The sadistic dungeon master Kiliban Nyrandil gives you a lesson in human suffering and he really loves his job. Remember that moment where you got to decide whether you should kill an innocent to join the Dark Brotherhood? To take that first step down a deadly and corrupt path towards evil?

Well here you don't get a choice and Kiliban instructs you to pick off a party one by one. Do you set a flock of Gnarls on the adventurers or summon one which grows to giant proportions? Do you flay another alive or trigger a fire trap? But it's the final one which is a doozy: you can choose to create an illusion which makes the survivor think he's dead. Returning him to his body, the shock drives him mad. You can't refuse to kill and maim; your only choice is how they meet their end: do you press switch A or B? It's in Xedilian that you get to piece together the new Gatekeeper, literally, from a pile of corpses. Yes, you finally have the chance to do a Frankenstein and build your own monster.

But a little DIY monster-making is not all The Shivering Isles has to offer. There are new sets of armour to be found and some really cool new weaponry. Chief among them is the Dawnfang and Duskfang, a single sword which switches names between the hours of day and night. Each time you end a life using the sword, a note saying 'Dawn/Duskfang has extinguished x lives' pops up and the sword is powered up when you've killed twelve beings with its blade.

Walking around the environments is always a pleasure

Achievement fans will be glad to hear that The Shivering Isles also includes 250 extra points to earn. During the two and a half hours I got to play the game, I unlocked at least three. They seem to range from meeting the paranoid and agoraphobic Syl, the Duchess of Dementia, to completing quests. The quest-givers shepherd you into new areas but there is of course the freedom to roam wherever you want, not bound by quests. Unlike in Tamriel, there are no horses to steal and all you can rely on is your feet. The walk to Crucible takes a good ten minutes but once you've discovered a new city or cavern, fast travel saves you some time. Walking through this new land is an experience all in itself though; the two halves are beautiful in their own spooky way and contrast perfectly to reflect Sheogorath's own hilarity and madness.

If you needed a reason to shun the outside world once again, then The Shivering Isles could well be it. It's shaping up to be a stunning companion to the original game and 20-30 hours of new Oblivion questing shouldn't be sniffed at. If you haven't already worked it out, I can't wait to play the whole thing through when it's released next month.