Eris has been working with Mara and Ikora since she escaped from the hellmouth. Although we only started to figure this out in HoW in the grimoire card where Mara talks to Eris about the dreadnaught, but if they knew about it, and presumably it's power, why would the queen attack it? There is also more than one dreadnaught, according to how people speak about it, so Eris obviously knows about these ships. Maybe Oryx's sister plays a part in this?