Breaking news: With the aid of his vast network of spies, the thiefmaster has ascertained the whereabouts of the secret base used by the mercenaries for launching the attacks. In fact, it seems like there is a base on both Trammel and Felucca facets of the world, likely to support the blackgates on the respective facets. In face of their recent defeats, the mercenary force is reeling and this is the perfect time to strike back before they can recuperate.
So, for a change, thiefmaster is going to provide you information, instead of asking you to figure it out for him. All you have to do is use it to access the base and destroy the source of the blackgates. Simple!
Note: check the calendar for the times. The thiefmaster or his agent will meet you at Trinsic moongate in Felucca for the Fel invasion and Jhelom moongate in Trammel for the Trammel invasion.