The Pirates Guild is a quest hub located far from Ythanica. You can travel to the Guild from spawn by talking to one of the NPC's. To return to spawn, simply turn around and walk through the portal.
The Pirates Guild is home to a multitude of eager NPC Pirates with different quests that you can take from them. These quests will individually reward you with Siren Fragments, which can be used to craft a Siren Catalyst to unlock the Siren Class.