If it exists, what happens if an assassin teleports himself to the bed chamber of the emperor? Can one teleport in/out of Oblivion and other dimensions? What sort of restrictions would there be?

There's also Almsivi Intervention and Divine Intervention spells. Almsivi Intervention teleports you to the nearest Tribunal temple and Divine Intervention teleports you to the nearest Imperial Cult (Nine Divines) shrine.

And don't forget the teleportation spell Quaranir and the other Psijic mages use.

Propylon chambers too.

Typically, the destination needs to be magically attuned, so an assassin would first need to mark the location somehow (Marking the Emperor's room would be difficult in and of itself.). Mark and Recall are spells not included in Skyrim, however they were available in TES 3 Morrowind.

Teleporting in and out of Oblivion is very possible and happens often throughout the Elderscrolls series (especially TES 4 Oblivion); however it is very difficult and requires sophisticated magic or special locations. Many of the realms of Oblivion have hostile environments, much like Mars or Venus would be to us in real life; travelers overcome this with protective magical barriers surrounding their bodies. You can read about it through an in-game book, http://www.uesp.net/wiki/Skyrim:The_Doors_of_Oblivion, while playing Skyrim or Morrowind.

Tens of assassins could mark a magically attuned location somewhere on roads that one day the emperor will travel. They can take him by surprise if they know the emperor is traveling on the road, right?

Teleportation in general sounds like a bit lazy plot device to me because almost all characters fail to use the ability to full extents. Why don't capable mage enemies just teleport away instead of falling to the main character's sword(King of worms, I'm looking at you)? Why don't they use it for more complicated plots and schemes? Can one sabotage a marked teleport location, making it dangerous for one to use that spot?

None.

But if you don't know already, Dark Brotherhood assassins are recruited based on how well they do killing people which usually means they don't come from a great background.

Great wizards however, like an Archmage or a Telvanni Wizard would be able to teleport themselves in many ways, take for example the Mark and Recall spells which were removed in Oblivion.

Yes, they could try and take him by surprise, but will they get past the guards in his entourage?

Some enemies in Skyrim do use teleportation to evade you and your attacks, but it is considered an advanced form of magic that a common mage doesn't have.

Sabotaging a marked location hasn't been an available gameplay mechanic as far as I know; however, considering the flexible nature of magic in TES, I think it is entirely possible from a lore-based standpoint.

Mage enemies fall so easily because it's a game, I never payed much attention to Mannimarco but in Elder Scrolls lore, as in Morrowind, great Wizards are capable of almost anything and wouldn't need to teleport. A simple thunderbolt through the Emperor's convoy would be simpler.

Typically the emperor would have wizards at his disposal as well. It may seem that simple, when you assume the Empire consists only of sword-and-board infantry; but that would be a mistake.

coc

OP: There are Intervention spells as well as Mark/Recall.

Mark and recall only existed in Morrowind, arena and daggerfall used fast travel, it was not like Oblivion "removed from the series" it only ever existed in one game in the first place.

Which only bring you to temples or shrines.

Uhh, go away! It was in Morrowind, not in Oblivion, thus it was removed. You're so annoying, I swear... Get on topic or something.

I was pointing out that it was not a staple of the series, they were created to replace fast traveling, which when fast traveling was added back, made the spell(s) useless. Calling me annoying is not going to get you anywhere either.

all of them have already been mentioned, the intervention spells, mark/recall, and i guess to some extent the Conjuration spells are sort of like teleportation, but only work in a specific way.

Would there even be Almsivi Intervention anymore?