You pick a side in a civil war, and help it win. Dragons show up about halfway through. Just as the war is won (with you as its most important general), you murder the king you were supporting and usurp his role, and attempt to use your new found resources to combat the dragons. Magical magic at High Hrothgar, and a deal with Mehrunes Dagon. Despite everything, Alduin slowly devours all the world, with the exception of the summit of High Hrothgar, where you and your allies gather for one final stand. This is a tripy, magical, OP boss fight where you face dopplegangers and people long since dead. Once they are defeated, you stand and face Alduin, but you cannot win. He has you pinned down, your allies are dead (except for Mehrunes, who has fled). In a final, desperate move prompted by some intuition, you reach up into your bloodied chest and pull out your own heart, flinging it at the foot of Alduin, proclaiming something along the lines of "for our shared madness I do this!"

Fade to white. Voices. The flashes of images. Numbers. The sounds of nature. Fractals. A high-pitched noise, growing louder. More images, flashing faster, until they overtake the white. The single beat of a single heart. Then another. It steadies, the images slow and fade into the summit of Hrothgar, unmarked by neither war nor age. At your side stands your shield-thane, Leaper Demon King, anon Dagon, now cleansed. Behind him stand your Companions (who number 500, if only in spirit). The fifth age is come, and it is the limit of this world, for it is without ending.

Conditional ending based on the deeds/crimes you did for each faction or town, kind of like the ending for Fallout New Vegas, where each factions ending or a character's ending was based on your actions throughout the game.

You realize your character has been dead the whole time.

TES V: Lost