I'm my headcannon TLDB is a redgaurd volkihar vampire named Akar Zet. Akar becomes a vampire because he sees it as the easiest means of immortality and the volkihar are the strongest vampires in the known world so his strengths always offset his weaknesses. He can even go out in the sun, albeit with great discomfort.
Akar wants to be immortal because he knows that Alduin will inevitably return to devour the world and the only being who can stop that would be Akar himself. The only means of being there to greet the great wyrm upon his inevitable return is to be an immortal and to be right there at the gates of sovergard waiting for the fucker to arrive.
Their struggle will be endless as Alduin is not strong enough to kill and devour TLDB who, while not invulnerable, won't die of old age and can't be surpassed by Alduin.
While likewise, TLDB cannot kill Alduin because while his power surpasses the dragon, Alduin is literally immortal and cannot be killed or devoured and will always, ALWAYS return to life.
Thus TLDB will fade into obscurity as he and Alduin engage in an endless powerstruggle forever with neither side being fully able to defeat the other and neither side being able to pursue other ambitions because Alduin will always return and TLDB will always be waiting to do his job. It is fate.
He never knows when Alduin will return. Unlike the first time he has no reason to believe he'll have a millennia long break. Alduin could return at any moment. All he can do in the meantime is feed on mortal blood and wait for his old foe to return. He sees no moral fallacy with this because he sees himself as the repeated savior of the world and thus any mortal he needs to kill to pursue his work is a worthy sacrifice.
Maybe one day in a far off future Alduin and he will talk it out. Or maybe they begin to look forward to their fated meetings and see their struggle as friendly sport. Maybe Alduin gives up and they become friends. Who knows. But in an ironic twist, their relationship will be the longest and most intimate that either of them will ever have with other living beings.
