What she does is not lethal, BUT it can be if she wanted to. Parch prefers not to kill her victims IF they are powerful. Since then, she can just wait till they gain their strength back up again after a long while and come back to feed again. :D
Her victims will be conscious through the whole process, they will feel their energy getting drained, they will feel dizzy at first, then all of their muscle will be spazzing and aching and somewhat choke as their magic mass spazzes out of their throats by force, which tears the insides causing bleeding. They will fall unconscious afterwards and have temporary memory loss for a few weeks or so.
Parch will make sure to erase her victims memories for that moment, so she can stay hidden.
but something tells me you aren’t looking for ways this could go wrong.