You can be the: Vestige, Last Dragonborn, Hero of Kvatch, Nerevarine, Hero of Daggerfall, Eternal Champion, Champion of Cyrodiil, Apprentice, or all of the above. You can choose where you are imprisoned out of the following areas: Cyrodiil, Skyrim, Imperial Province, Morrowind, or any other province in Tamriel. You can even choose some planes of Oblivion, such as: Apocrypha, Ashpit, Attribution's Share, Coldharbour, Colored Rooms, Deadlands, Evergloam, Hunting Grounds, Moonshadow, Scuttling Void, Pits, Misty Grove, Shivering Isles, Spiral Skein, and Quagmire.
After you escape, there will be a quest-line sending you either to a shrine of Stendarr or follower/ priest depending on what realm you are in. You will be informed of the Dark Brotherhood, an organization developed from former Assassins of the Morag Tong. You will also be ordered to exterminate them, as they could be much more successful than the Morag Tong. As you arrive from your journey to Vvardenfell, you will be faced with a choice. Destroy the Dark Brotherhood or join them. Either way, you will meet Ezio Auditore da Firenze, and depending on your choice, he may be hostile or friendly. If you choose to destroy it, (and Ezio) you will get a courier message from the Templars. If you choose to join the Dark Brotherhood, he will teach you how to move, fight, and be like an Assassin. He will also teach you to use everything he learned in life, such as the hookblade or the hang glider.
There will be quests from the Templars, the Dark Brotherhood, and the Assassins.You will have the option of turning as a werewolf, wereboar, werebear, werebat, werelion, werecrocodile, wereshark, weredaedroth, and any other were-critters.
Mounts will include, but will not be limited to: Daedric Titan, Spider Daedra, Ogrim, Giant Snake, Dreugh, Wamasu, Ogre, Thunderbug, Bone Colossus, Lurcher, and standard horses. (All mounts can be customized)