3. The awe-inspiring fight against a Legendary Dragon
The Legendary variety is the toughest breed of dragon found in Skyrim. In combat they can use the Fire Breath, Frost Breath and Drain Vitality shouts, in addition to a powerful bite attack. Drain Vitality in particular can quickly deplete a player's health, stamina and magicka if caught in the blast.
In terms of appearance, Legendary Dragons have horns similar to Durnehviir, although they only have two as opposed to the undead dragon's four. Should you manage to get close enough you can see the Legendary Dragons also have unique eyes, with four pupils being located in a single socket.
Encounters with Legendary Dragons are rare, and although some players have reported seeing them at lower levels they don't usually start to appear until level 78 or above is reached. They are still encountered randomly even at these high levels, although fast travelling to Dragon Lairs and other locations dragons are usually found, such as The College of Winterhold, can encourage them to spawn.
If your character is below level 78 and you're having trouble finding a Legendary Dragon, you could use the Ogham Infinium exploit to rapidly develop to the required level, as this trick apparently still works with bookshelves.