Just beat a Legendary Dragon on Master Difficulty.
Fuck yeah. Taking the guy down took some luck, some skill, and some friends in high (or, more specifically, low) places. Minor Dawnguard spoilers ahead:
The character I play with now has had quite a journey. He started off as a main quest playthrough. I finished Dawnguard with him, but after that didn't feel the drive to go back and do the main quest. Part of me wanted to create a new character so I could do the Main Quest and Dawnguard questlines in order. But, since this guy was in the middle of some quests, I quickly realized I could use him to farm some achievements: specifically the Vampire Mastered and Legend ones.
So I did the one thing I had never done before in this game: I set my morals aside and deliberately became a blood-sucking creature of the night. As a Vampire Lord I traversed Skyrim, littering its streets with the drained bodies of the dead. I became powerful. I became hated. Wherever I went, the sun would set. And at last I unlocked my final perk: Achievement Unlocked.
That still left the Legendary Dragon, though. Thorvild the Wanderer, however, was only a measly level 37 warrior. The aforementioned dragons only appear once you hit level 78. So, once again, I did something I had never intended to do with this character: I acquired the Oghma Infinium and boosted my skills until I reached level 81. I decked myself out in the appropriately evil-looking Heavy Falmer armor, forged a dragonbone sword and went hunting. But the Legendary Dragon, it seemed, was an elusive creature. Everytime I reached a new dragon lair, I would find a low-leveled, pitiful being I dispatched after only a few blows. The only challenge I encountered was an Ancient Dragon, but it, too, succumbed quickly to my overpowered sword arm.
Eventually, I had only one dragon lair left on my list. Ancient's Ascent, near the Cyrodiil border. One of the more hidden lairs, but thankfully I had already discovered its location during my earlier roamings. I fast-traveled there and arrived in the dead of night. Ahead, I noticed a dragon sitting atop a Word Wall, steadily keeping its gaze on me. Unfaltering, I stepped forward and let loose a Soul Tear shout. The beast, of whose breed I wasn't yet sure, surged forward and hovered menacingly above me.
At that moment, I noticed the horns and the dark veined wings. This was my target. This was a legendary dragon. I leveled Spell Breaker, warded off its Drain Vitality shout, and took cover behind a piece of Nordic architecture jutting from the mountainside like an old bone. I fired off pot shots at it with my crossbow and got close whenever it landed near my hiding spot. I saved a lot: exposure to the dragon's fire breath meant instant death. After twenty minutes, I had only whittled down a quarter of the beast's health bar. I needed backup.
Luckily, I knew just the guy. I waited until the dragon took flight, aimed at the ground and shouted "DUR NEH VIIR". True to his word, the ancient undead dragon sprang forth from the Soul Cairn and appeared beside me. The Legendary Dragon quickly took notice and prioritized targets. As the two dragons did battle in the air, I loosened every crossbow bolt I had at it. Occasionally, the dragon would retreat to the ground and engage me, only to quickly take off when he realized I, too, had teeth. Durnehviir called forth some minions from the Soul Cairn as he forced the Legendary Dragon down a final time. I charged forward, plunged my sword deep into the Legendary Dragon's mouth...
Only to get roasted when it breathed fire at me. Reload.
I charged forward again, but this time kept Spell Breaker aloft. Bashing the dragon's face aside every time it tried to barbecue me, I eventually managed to drop its health bar to zero. As it let out a final roar, its flesh burned off its bones and its lifeless body slid downhill. I absorbed its soul. Durnhehviir landed beside me, shouted in triumph, and then retreated back into the Soul Cairn. I restored my health, gathered my loot, and took off into the night.
I know I didn't need to beat the thing on Master to unlock the Achievement... But heck, it felt good.