Each time you attack pirates, the pirate faction's anger towards you will increase. You can view the current level of their anger by tapping on any pirate on the System Map and selecting View. When their anger fills to the top, the next attack you perform on any pirate will cause a countdown to begin. At zero, revenger pirates will directly attack your Space Station. When defeated these pirates drop great rewards. Try to make sure you have your fleets docked before the attack to ensure you can fight the pirates off with minimal losses. Revenger pirates can only be triggered once per day.