The Sanguine Sirens are a band of outcast pirates, led by Leanna Gadreel. They banded together to survive the harsh seas, taking what they need to survive. To join the Sirens you need to sign an unnatural contract with Leanna, who will protect you from harm, as long as you vow to do the same for your comrades.
In actuality, Leanna (who turned out to be a Sea hag) cursed everyone who signed the contract to do her bidding without question, ultimately erasing any free will left. In some extreme cases they gave up their bodies as well, mutating them into strange monsters.
When the crew of the evernoon killed Leanna and in turn sank Whale Island, everyone who signed the contract collapsed and sunk with the island.