After untold hours exploring the realm of Daedric Prince and general nasty guy Mehrunes Dagon, it's easy to forget that just because his plane of Oblivion is reminiscent of Hell that the realms of his brothers and sisters might be equally different. Such is the realm of the oddly likeable, yet still insane, Sheogorath. The Shivering Isles might evoke images of an icy tundra but it's actually anything but. Half is almost pleasant, bathed in autumnal light, and the other is filled with mist and suitably creepy. Welcome to the first and last expansion for Bethesda's award-winning RPG The Elder Scrolls IV: Oblivion.

Although visually very different, The Shivering Isles feels very much like a part from the original Oblivion; a journal note pops up directing you to Niben Bay, south of the Imperial City and an aptly named 'Strange Door'. A mysterious island has appeared slap bang in the middle of the lake and there is a strange portal at its centre. You don't need to be a high level to enter or worry about a different control system as The Shivering Isles uses exactly the same game engine as Oblivion.

Walking through the portal is as mundane an act as they come but after a quick civilised chat, Bethesda surprises everyone with a truly stunning sequence which looks equally impressive on both PC and 360. The room in which your character sits turns into a swarm of blue, green and purple butterflies (and that one moth) which flutter away to reveal the Shivering Isles in all its odd glory. Sheogorath might be nuts but sometimes even the insane Daedric Prince can summon up things of incredible beauty.

The Shivering Isles is vast, with its own dungeons, hamlets, villages, marauders and wildlife. It's also self-contained and you don't have to return to Cyrodiil until you really want to as everything needed for the next fifty (and then some) hours of play lie in Sheogorath's realm. He might hate horses but that doesn't mean you shouldn't wander aimlessly, taking in the landscape. There are stunning waterfalls, mountains, caverns, hidden caches of treasure and of course, locals who are just that little bit bonkers; some are nearly normal, some are religious zealots and some just love to flay the skin off anything that crosses their path.

The first task is to go see the Mad God and the only way to do that is by downing the Gatekeeper. Previously, it was a simple matter of hacking and slashing your way with a good enchanted weapon and as much magic as you can muster. Now there are a few choices, you can hack and slash but The Shivering Isles' version of Frankenstein is insanely hard to down that way. Instead, there's a quick quest to complete and some enchanted bone arrows to earn.

Sheogorath waits for you in his city, New Sheoth, and getting there is entirely up to you. Do you take the northern route or southern route, each with its own different challenges? Whichever you choose, unlocking the door will give you a nice new spell to use on your travels. As the game progresses you also learn more useful enchantments which include the ability to summon Haskill should you feel inclined, mess with the weather or teleport back to Sheogorath's palace for some emergency healing.

As with the main game, there is an over-arching quest line which deals with the mysterious Greymarch and the Daedric Prince of Order, Jyggalag, with the unpronounceable name. This quest line sees you learn more about Sheogorath, his realm and the personal bickering of the underlings who watch over his realm, drug addict Thadon, the Duke of Mania and paranoid Syl, the Duchess of Dementia. But there are also numerous mini-quests which see you helping out locals, torturing anyone who takes your fancy. It's a lot of fun and the new assortment of odd NPCs is a refreshing change to those found in Cyrodiil. While the actors from the original have returned, there's plenty of new information to be had, new weapons and books to read.

Butterflies, butterflies and more butterflies.

One of the best weapons is the mystical sword whose name depends on the time of day it is used; making it either Dawnfang or Duskfang. Each time you take a life using the sword, a note appears informing you that 'Dawn/Duskfang has extinguished x lives'. Twelve kills then yields a powered up weapon with a lot more clout.

The main quest line also allows you to use the skills learnt back in Cyrodiil, making the whole line feel like a blend of the quests completed in the Thieves' and Fighters Guild with a touch of the Dark Brotherhood thrown in for good measure. This is a nice touch as variety is the spice of life, especially when The Shivering Isles could have been just more of the same. Instead, it's different and just as engaging as the main game. The drastically different environments are filled with new enemies and generic caves to explore. Even after completing the main quest there's still plenty to do and, failing that, there's always the option to return to Cyrodiil with your new uber-weaponry.

The Shivering Isles is the perfect expansion; it gives you the chance to be good and become Sheogorath's Champion but also lets you be periodically evil, killing and torturing, you can even be the Oblivion version of Frankenstein and create you own monster. The land is as strange as its creator but beautiful in its own way. Indeed, Cyrodiil looks almost pedestrian in comparison. While it's a shame that this expansion is the first and last, Oblivion's swansong was well worth the wait and fans of The Elder Scrolls will not be disappointed.