I think turning to the "light" side could be a unique way to power up a v.

Instead of doubling down on being a creature of the night (and using the powers he gained from turning down Valhalla) he should oppose the darkness and reject those ideals, and thus gain the powers of "light". A.k.a. keep the non-dark powers (staying young, being very strong and fast) but instead of things like bats, shadows, blood, etc., now he has a special connection to nature:

• helps and respects animals, and in return animals always seem to avoid conflict with him
• knows when nature related things will happen, like tree branches falling, clouds moving away and the sun/moon shining through, rain stopping, where its safe from striking lightning, etc
• feels where the water in the river is flowing, what rocks it brushes against in the distance, how many words and laughs it hears passing through the next village; etc.
• hears and feels the wind, how it moves through objects and people as they move, can predict not just passages and animals/people coming, but can anticipate their moves perfectly
• knows plants and materials like touching them gave him the story of their lives, thus using wooden or metal weapons with such mastery no human had ever known

So, basically an elf on steroids, minus healing powers (but you can totally give him healing powers too, if you want)

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Imagine this: he has a small adventure/encounter of some kind, and realizes how disgusting dark creatures like him are, he decides he will never be like that, not yet fully coming to the conclusion that he will sooner die than give in, but still firm. Then you give him one or two adventures/encounters where this resolve is tested and strengthened again (repeated examples of what he doesn't want to become (both literally, and metaphorical -> he meets/overcomes small (unimportant, human) people with evil traits like greed, bloodlust, cruelty, etc,), and he helps people/defeats bad guys without his powers (first just because he doesn't need them to deal with some bastards, then because he doesn't want to use them), then he fully commits to this lifestyle (not giving in to his urges, and not using his powers) and vows to keep it, knowing what it will inevitably result in (him perishing).

Then, he is fighting a bad guy (really bad for the people he is making life hard for, but a small fry for him, someone he could defeat in a second with his powers), but the bad guy is getting the better of him since he is weakened (hasn't eaten in months). He is on the verge of death, he fights and fights, every blow reminding him and the audience that it would only take him a lift of a finger if he used his powers, but he doesn't; he is taking a blow after blow after blow... ,and the question becomes not how he will stand up and hit back, but how he will NOT hit back as every inch of his being is screaming at him to use his powers and kill the bad guy. He grinds his teeth and clenches his fists, pouring in every last drop of his strength to not hurt the guy, the color in his eyes - like a reflection of his being - starts cracking like a gem, cracking more and more with each blow he receives, the bad guy noticing it, and gaining a new, primal sense of drive and hitting "the monster" even harder, the eyes cracking and cracking more, resolve raging in trembling fists as he tries so hard to hold himself back as he is on the verge of death, and then... he succeeds.

With one final blow, the world goes dark around him, and he goes limp.

The bad guy murmurs profanities at the "monster" as he tosses his body (he's been holding him by his collar) aside and leaves unceremoniously, wanting nothing to do with him, or his murder.

Silence, as our man lies motionless on the ground. Silence, as if there's never been a fight here just minutes ago; as if the body, and the stepped-on flowers, the torn-out grass and axe-bitten trees all were just a natural part of the forest; not an abomination, a mark of evil left by people too evil to care.

As if by called, the dark clouds above (been there all afternoon, our man even remarks at them before his fight with the bad guy) finally release their tears. It starts to rain. Drop by drop, washing away the scene, drop by drop, as the thirst of the ground is filled, slowly darkening to the tune of dancing rain, drop by drop, as the dirt begins to melt away from the man's face, drop by drop, as his half curled up fist is hammered by the water, crack by crack as his eyes continue to break under the weight of drops, crack by crack, as the armor of shadows beneath his skin fractures and breaks, crack by crack, as his-

"Hhhh!" he inhaled sharply.

--> and he is what's described above. You can have a whole lot of fun as he discovers his abilities, and it's also a good way to keep tension in the story (you can have him always deal with antagonists/antagonistic forces that are just around his current power level, so they pose a threat and are not easy to overcome, plus you can basically keep leveling him up, giving him more power and bigger challenges at each step until he reaches peek and faces your fire-lord friend)