The Madgod is falling and needs a new warrior to fight his evil foe... Order!

Go through a portal and appear in a completely new realm full of fantastic creatures and shocking horrors!!! And find out of the secrets that keeps the world of the Madgod in constant struggle between chaos and order!

In this new explansion for Oblivion, find an entirely new world in which to gain powerful weapons and abilities!

This is loaded with extra goodies! There is the main quest and many little side quests! Also when you gain 'power' (dont wanna reveal anything) you can control so much cool stuff like the weather for a set time... its ace!!!

This is the first full featured expansion to Oblivion, and IMO it leaves a lot to be desired. You have a main quest that is more or less fine, what is lacking is the rest. Most side quests are simple or even contrived, people inhabiting the land of madness do not look so mad after all, and there is not much variety in the new lands.
Still, it deserves its price. Too bad they didn't take the chance to improve the original game interface, that still limits you too much. Some examples? The very few quickslots are not enough to cover all you will probably want to use: magic, different weapons, various items. So you'll still need to pause the game to quaff a potion, pick up a particular weapon or select an unusual spell. This breaks up immersiveness a lot, at least for me. Oh, and it looks like you still cannot delete spells unless you mess with saved games and the console. Shame on Bethesda for this one!

Expansion is not just added on things. It's almost a whole new world. Massive with new monsters, weapons, characters, missions and of course places to explore. Worth it.