The Sea of Rakash is a location that features in He-Man and the Masters of the Universe.
During ancient times, the Sea of Rakash did not exist until Hordak envoked the Spell of Separation at the Well of Darkness which created the Dark Hemisphere on Eternia. This act led to the Caverns of Rakash being transformed into the greaty Sea of Rakash which was located near the Mystic Mountains that was home to Avion and Andreenos. In later years, the villainous Evil Warrior later known as Mer-Man resided within the Sea of Rakash as well as commanded the various creatures of the deep ocean. When the Mystic Wall was created, Skeletor sought a means to bring down the magical barrier and tasked Mer-Man with finding the coridite crystal that would serve as a potent power source needed for the task. Mer-Man eventually succeeded and brought the crystal to Skeletor which was used to bring down the Mystic Wall.
Afterwards, King Hiss deciphered the Tablet of the Elders and learnt that the Serpent Ring had been hidden within the Caverns of Rakash located now within the Sea of Rakash. This led to him leading his Snakemen into the waters from Subternia where they recovered the ancient relic after battling the Masters of the Universe.