The pixie queen tells us that the pixies are overwhelmed with anger and sorrow, to the point of mindlessness. So what is more likely in that scenario, that they throw themselves at the enemy till they die (likely killing several in the process, a pixie with razorblade wings and covered in thorns is no joke, Richter backed off from the one he faced for a reason. Imagine a manuevering, flying knife that can think, and you have a pixie in warform) or fly off to find something to bond with? Besides the Dungeon cannot create pixies regardless of how many it might consume. Just it cannot create rock giants, dragons, mountain trolls or gnomes. They are not in the animal/beast category. I see plenty of problems with the Dungeon being able to syphon magic from a weakened adventurer/invader from a mechanics perspective.