The Ebony Warrior (quest)

Walkthrough
Upon reaching player L|level 80 the Dragonborn is automatically approached by the Ebony Warrior in one of the major cities in Skyrim, e.g. in Whiterun.

Talk to the Ebony Warrior
The Ebony Warrior will challenge the Dragonborn.

Meet the Ebony Warrior at the Last Vigil
And then convey the location for a meeting place at Last Vigil, a small camp up the snowy mountains north-east of Fort Greenwall.

Tip: Apparently this is the strongest opponent in Skyrim, so come with a follower. Since the Ebony Warrior does not attack of his own accord, a powerful bow and a sneak attack could quickly take him out.