The Myrka is a very large water-dwelling creature which lives in the deepest depths of the ocean. The Silurians altered its biology, turning it into a highly destructive weapon. Its large head, with white eyes, swept-back Sea-Devil-like fins and fronds fluttering down its throat, sits atop a long neck which leads to a wide, squat body. The Myrka walks on all fours, but can raise up to two legs if it wants to leave its clawed front legs free. It also has a long tail and scaly green skin.
The Myrka is equipped with an electric shock powerful enough to kill a man, and is transmitted through contact with its skin. The Myrka can also conduct its electrical charge through metal walls, killing anyone in contact with them.
As a result of its deep-sea life, the Myrka has a strong aversion to light, and no tolerance at all for ultraviolet rays. The Doctor is able to turn this to his advantage - he temporarily blinds it by hurling a weapons cartridge at it, which explodes when it comes into contact with the Myrka's skin. The Myrka is finally killed when the Doctor directs a high-intensity beam of UV light at it.