My favorite battle of my campaign

My favorite battle I've run in the campaign has been the boss encounter at the end of the second arc of my campaign. Arc Two, which had run about fifteen sessions, investigated why sahuagin were attacking coastal villages. They discovered the sahuagin were worshipping a forgotten being from the depths, and had taken on its madness. The players eventually sought out the protector spirit of the ocean - a nature spirit in the shape of a massive whale, who turned out to be the mother of the forgotten being - another whale spirit that was cursed with apocalyptic rage. The party agreed to help the mother whale by protecting some of her orca followers, who needed to sing a lullaby to put the whale-son to rest. When the battle was met, the son was accompanied by a horde of sahuagin worshippers and attack sharks. The setting was a sunken ship over a deep black chasm. The mother whale engaged her son in melee, which raged all around the battlefield. This was just a distraction: the true goal…