I have not heard of "Drow Death Elves" before... Drow+Vampire hybrid, that's a first for me.
Of course, I feel the only good Vampire is a dust-bunny, so I'm a bit bias. But, I do like the idea of a slave!Drow. Rorschach's Blot's D&D fic (Odd Ideas 133 "Dungeons 'n Drow") was the first to introduce a Drow with Harry, and I like it. And it works so well.
That this "Death Elf" also takes the form of a weapon for Harry is original, a living, shape-shifting rune weapon will be interesting to see more of. Especially, as I can't see Dumbledore wanting it around.
Side interest: what "desires" will this Death Elf have for her Master? And will others learn of her?