Viinturuth

Viinturuth is a named dragon found in the wilds of Skyrim. He speaks in native dragon tongue when in the presence of other dragons. He is aggressive and uses fire or ice, depending on your level.

He is often found in the company of Alduin, who will not join the fight but rather fly off to let the Dragonborn and Viinturuth fight.

Viinturuth can be found being resurrected by Alduin just West of Anga's Mill, by the roadside.

Bugs
Found him being resurrected by Alduin, almost killed him but took off intook the air. About half an hour later missing him he went back to his burial mound where I 'killed' him but did not die or react to my attacks.
 * After defeating Viinturuth it is possible that he'll attack you again and may be invisible. (PC, PS3 and 360)
 * When you encounter him Alduin may not finish resurrecting him. He will crawl out of the grave but remain a skeleton and he will not regain his flesh. He will not attack, but you can still kill and loot him and absorb his dragon soul. A column of the dragon soul energy stays above the mound as well. (Happened on Xbox 360 & PS3)
 * While playing through, Alduin was found resurrecting him, but Viinturuth was stuck in the ground, fully resurrected. He was just stuck, he couldn't attack, and could be "killed". His health bar was fully drained but his name was still on the top of the screen and he was still moving a bit. Nothing could get him out of the hole, no amount of dragon shouts or staffs would work. After leaving, using "Wait", and returning, Viinturuth was found on the way back. (Killable and lootable, with an absorbed soul.)
 * Alduin attempts to resurrect him but the scripts are slightly off and the sequence doesn't perform correctly. First Alduin says his lines, then use his force voice but is hovering in the wrong spot and misses the burial dome which doesn't crack. Viinturuth then crawls out of the ground fully formed skipping the step where Alduin brings him to life from the skeleton. The script seems to be broken and reverting to an older save game in Windhelm and travelling again to the dragon burial site to restart the whole sequence doesn't fix the error.
 * Found being resurrected by Alduin and he attacked, but once his health was low he wouldn't land. "Killed" in the air, but would not die or land.
 * Similar thing happened to me. Found him with another dragon, killed the other one and then Vinturuth was left. Attacked until his health bar was gone, but he kept flying around. Eventually I went to a big open space and he landed, and died as he did. Not sure if this was because I was in a narrow place with trees on either side, of because Vinturuth is bugged.
 * There is a chance of encounter this dragon invisible, but still attacks and can be attacked. After defeating him, no dragon souls are obtained, but he can be looted as usual, although he is still invisible after been killed. Dragonrend reveals his position via the blue glow effect around him. Also, Aura Whisper can be used to track the dragon's position during the battle. (Confirmed on Xbox 360, PS3, and PC).
 * Found him after main quest was completed, on the way to Fort Kastav, just North-East of Bronze Water Cave.
 * Found him attacking Windhelm. It played like a normal dragon fight, could be killed, and soul absorbed.
 * Found south of Windhelm during the main quest. Attacking farm and fighting ice wolves.
 * Appeared after finding Pale Stormcloak Camp and looting a chest, killed and looted with soul absorbed. (This was after the quest "Dragonslayer")
 * Found him near his original location, but stuck in the ground. His upper body was above and the rest of him stuck under the ground. Could not be killed, though his life bar depleted yet only showed his name. There was no way to absorb his soul. Later when I had proceeded to "Anga's Mill" Viinturuth appears behind me dead. I was then able to absorb his soul and loot him as normal.
 * Encountered while fighting Alduin in Sovngarde, killed, looted and absorbed his soul as normal.
 * Encountered near Anga's Mill along with an Ancient Dragon. Was able to kill both dragons and absorb their souls as in any other dragon encounter although all of Viinturuth's body, with the exception of his head and upper neck, clipped through the ground when he was killed.
 * After defeated, his corpse may be found inside Windhelm, an it can be looted. However, his soul cannot be absorbed.

Encountered him after I had found all 30 crimson nirnroots and left blackreach. Was able to defeat him, absorb his soul, and loot him.

Found him dead in Whiterun and was able to loot him, but not absorb his soul. He lags around the town and will not go away.