The old Prima guide says it is called Proof Of Clan Alliance. The starting NPC is Sir Kristoph Rodemai in Giran. The map indicates he is in one of the shops/guild near the east entrance of Giran. That quest requires a lot of coordination among the different players, as the clan leader will need to drink poison at one point and be kept alive while other players are doing their part. I'm thinking it will be easier if you can find a guide before you start the quest.