I was vamping around Karthspire with Alduins Wall quest active. Interesting fight, used Esbern and Delphine as 'healing potions'. I was ignoring the dragon and focusing on the Forsworn. Went inside to regain stamina, those two followed so I went back outside.
Night had since fallen, so I reverted and turned my attention to the dragon when I realized it said 'Legendary'. I thought I had my dragons confused because I wasn't level 60, but when he expired, the achievement popped.
Maybe he knew that I had two crafting skill maxed... --- JAFO