Had a thought and thought I should check if anyone else's brain would have the same thought. If you were law enforcement-adjacent and you found a kid who a) didn't look a thing like his supposed relatives, b) is obviously being mistreated, c) is made to do all the chores around a house, and d) doesn't have a paper trail to the point that it's like he just appeared from nowhere one day, what conclusion may you come to?
Okay so this technically isn't in the spirit of "one-off" scenes because it's a continuation of another one (https://www.reddit.com/r/HPfanfiction/comments/1s21s2l/dhampir\_harry/) but I have no idea what happens next and I needed to test this out somewhere.
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According to folklore dhampirs have a tendency to become vampires when they die. This isn't true, if it was then Harry wouldn't be as short of relatives as he was, but they are harder to kill, at least if the will to live is strong enough. As with the creation of vampires it ultimately comes down to a choice; survival vs humanity. Although as a dhampir was never truly human the choice doesn't have to be a conscious one, it doesn't require the involvement of a vampire or a pool of blood and it's more a matter of shifting the direction in which they're leaning than a transformation.
As the basilisk venom did it's deadly work, Harry wanted to live. To stop Tom, to save Ginny, because he was twelve years old and he hadn't even really had a life yet so he certainly wasn't ready for it to be over. And so deep within him something that had been passed down the last few generations stirred awake and pushed back against the encroaching venom.
Under most circumstances it wouldn't be enough, basilisk venom being what it was, but on this occasion it had something to work with. Blood heals and strengthens vampires because it is the currency of the soul, so while not ideal, a piece of soul available without blood would do in a pinch. Even better, it was connected to another much larger piece of the same soul close by whose housing Harry was currently in the process of destroying and so was promptly reeled in.
It wasn't a whole soul but it was enough of a one to use to beat back the effects of the venom, although given the circumstances Harry barely noticed that he suddenly felt better. Within him a precarious state of equilibrium between the two opposing forces was reached, one that wouldn't last for long. Fortunately the addition of phoenix tears to the mix meant that when it ended it did so in Harry's favour.
But a portion of his ancestry was no longer dormant and what it had done to try to prevent his death had left its mark. Harry walked out of the chamber somewhat less human-like than he'd been when he'd entered it.