I had one instance in Skyrim where 2 legendary(Ancient?... the toughest ones in the vanilla game) dragons and a Dragon Priest all appeared at once. Poor lydia didn't stand a chance. It was out a word wall, I had just arrived when the randomly spawned dragon appeared, while the other dragon took off from the Word Wall... I ran to the word wall hoping to use its structure as cover from their strafing attacks, but found a Dragon Priest bursting out of its coffin when I approached it.
After around 20 minutes of me chugging potions and liberal use of the Ethereal shout, I finally succumbed.
Epic fight, but I was fucked... I couldn't even run away.