

They won't appear at any Dragon Altars, nor will you randomly encounter one through your travels. After leveling up to 78, I was traveling on Shadowmere, and I suddenly saw a Dragon flying around and automatically assumed it was a Legendary Dragon! So I made a save, rendered the Dragon using the Dragonrend Shout, and it turned out to be a Revered Dragon! I was so pissed off! But one piece of valuable advice that I can give for anyone who's still looking for a Legendary Dragon to unlock the achievement/trophy is that you definitely have to be at least a level 78 to encounter one, and they can only be found in certain areas. Level 78 is definitely the magic level to encounter a Legendary Dragon, because with over a 1000 hours of gameplay, I never encountered a Legendary Dragon. I kept encountering Revered Dragons, which was cool, cause it was a new Dragon type to fight, and they're a lot tougher than Ancient Dragons, but I never encountered a damn Legendary Dragon?!?!?! So I started fighting differently, using Light Armor, Magic, Pickpocketing, anything that would level me up.

I have over 1095 hours logged into the game, I've played the game on Master the whole time, and had over a 100 Dragon Souls, with ever Shout unlocked and completed. When I completed Dawnguard, and unlocked all the related achievements, for both Vampire and Werewolf, I saw the achievement for "Legendary Dragon".

I struggled for quite some time to find a Legendary Dragon myself! When I downloaded the Dawnguard DLC, I think my level was roughly in the high 50's to low 60's, because I made the critical mistake of getting stuck in a certain rut of fighting, using only Heavy Armor with Two Handed weapons for majority of the game, which is why I didn't level up much.
