Marsissus

A Chief Sea Devil, described by the Thirteenth Doctor as a "zealot", led a raiding group of pirate Sea Devils in the waters of China in the 16th and 19th centuries.

He displayed many powers beyond those of an ordinary Sea Devil, including the ability to spontaneously teleport by turning into a fine green mist, to leap great distances and to control the Hua-Shen, a huge sea-dwelling creature. He used a special necklace to communicate in a voice intelligible to air-breathers. He intended to use the Keystone to flood the entire Earth, wiping out humanity and retaking the planet for the Sea Devils alone.

In the 16th century
He was awake as early as 1533, when he learned that the Keystone, a Sea Devil artefact of "infinite power", had become part of the lost treasure of the Flor de la Mar and entered the possession of the "great" Captain Sin Ji-Hun. He contacted Ji-Hun, who pretended to pledge himself to the Sea Devils to buy time for his crew to escape with their lives. After Ji-Hun "forced" his crew to desert the ship non-lethally, he offered it to the Chief Sea Devil. Deciding to become a feared pirate and strike fear into the hearts of humanity, the Chief Sea Devil decided to take over the ship, adapting it to be able to rise in and out of the ocean and even to fly, instead of sinking it. (TV: Legend of the Sea Devils)

Betrayal and imprisonment
Unbeknownst to the Chief Sea Devil, Lei Bao, Ji-Hun's second-in-command, was carrying the Keystone when he escaped the ship. When he realised that he had been outplayed, the Sea Devil decided to take his anger out on Ji-Hun himself, taking him back to his underwater base and forcing him to survive for centuries in painful suspended animation. Lei Bao, meanwhile, was eventually able to use the Keystone to capture the Chief Sea Devil and freeze him in time in the form of a statue on the shore, over which Lei Bao and his descendants kept watch. Meanwhile, his crew returned to hibernation in their underwater base. (TV: Legend of the Sea Devils)

Awakening and death
In 1807, Zheng Yi Sao, also known as the Pirate Queen "Madam Ching", who was seeking the lost treasure of the Flor de la Mar, learned of the statue, believing it contained the way to the location of the shipwreck of Ji-Hun's ship. Hacking into the stone-work, she broke the containment, freeing the Chief Sea Devil who promptly slaughtered most of the nearby village. The Doctor's TARDIS was drawn off-course and the Thirteenth Doctor, Yasmin Khan and Dan Lewis confronted him almost immediately. Though they were unable to subdue him, they forced him to retreat as he wordlessly summoned Ji-Hun's ship and sent a signal to reawaken his crew.

He directed the Hua-Shen to capture the Doctor's TARDIS from the ocean floor and then took the Doctor and Yaz prisoner. However, the Doctor was only briefly able to bluff him into thinking that she held the Keystone. After the two time-travellers escaped with Ji-Hun, and made their way to Madam Ching's ship, the Chief Sea Devil managed to acquire the Keystone from Ying Ki, Lei Bao's last surviving descendant, who had been wearing it as a pendant. He teleported back onto Ji-Hun's ship and sent the Keystone down, but did not leave to see the next phase of his plan as Ji-Hun himself put a sword through his heart, deciding to show "no mercy" to the creature who had taken "[his] ship, [his] crew and [his] life". (TV: Legend of the Sea Devils)

Behind the scenes
Although used in production documents, the name of "Marsissus" for this Sea Devil is never spoken in Legend of the Sea Devils, and Craige Els was credited as playing simply "the Chief Sea Devil".