The Aigle d'Argent was pirate Michel de Grammont's ship. When de Grammont became the admiral of the pirate fleet, his ship became the fleet's flagship.
During the Battle of the Hesperian Gulf, the Aigle d'Argent was comparatively undamaged, owing to de Grammont's refusal to engage the allied fleet directly. That night, after the battle, the various captains met aboard the Aigle d'Argent to discuss their next move.
In OTL, Grammont's ship was called the Hardi (French: Audacious).