Just participated in this little event, it was pretty cool. Basically if you see someone shouting at camp that the Garleans are coming....well they are lol. Look around for them gathering in a certain area and speak to a nearby NPC who asks you to help. You can't engage them in combat until you do. After killing enough or a certain time you emerge victorious and a chest pops at the NPC you talked to to start with deaspected crystals in it.