The Pest Queen is the leader of all pests. She decides what strategy pests use during one of their assaults, and she is summoned and controlled by the evil Wizard Grayzag. Due to her, pests always attack the same. E.g. ravagers are the front line and clear the battlefield, while leeches stay back and heal wounded pests. She is summoned by Grayzag through a portal in the Void. Usually, the Queen would be instantly defeated by Valluta, but she cannot fight due to being under the effect of "foodgoo". It is therefore up to the player to kill the Queen instead.
After her death, a new Pest Queen was born in the Void to lead the attacks in the Southern Seas, although the weakening between worlds is not large enough and Grayzag is not present (incarcerated or dead) to aid her, not making her a direct threat, according to Valluta.
If you right-click the Pest Queen right as it dies, it will show her pre-EoC combat level of 599.