Sea spawn

Sea spawn were humanoids that were transformed into a hybrid marine by making a pact with a powerful marine entity, such as a dragon turtle, a kraken, a marid, a morkoth, a sea hag or a storm giant, or by suffering a curse from one such entity. Once transformed, the sea spawn became a minion of one such creature, and could never stay away from the sea for too long.

History
It was said that the human inhabitants of the Purple Rocks were claimed as infants by the kraken Slarkrethel, and became fanatics dedicated to him. They grew up as humans, but became sea spawn when they reached old age to return to their master. The cult was tended by kraken priests, who were mostly natives, but could also include merfolk, merrows, and sea elves.