It's not necromancy. When she kills them they don't die, they just get trapped in her sword so she can summon them in battle later.
Mighty Magneto says:
"i think summoning and conjuration already fall under other powers."
I was going to say they would fall under magic, but there are people (like Katana) who only have summoning, not other kinds of magic. It could be it's own thing. We should probably find more characters that do that before it gets created as it's own power though.
I was replying to the original post before, but maybe what you're describing would fall under energy absorption, soul absorption, siphon lifeforce, or siphon abilities. I created a page for the Morlock MeMe who literally absorps other people, had no idea what power to use to describe that. http://www.comicvine.com/meme/41847/