Myrka were reptilian creatures with thick, rubbery skin, four legs, two arms, a long tail and a large head. They were taller than a human and much longer than a horse. They had wide, pale eyes and a large fin on their heads. Under their chins and around their bodies they had strange kelp-like growths. Myrkas were well adapted to deep-sea life, with large webbed feet, a flat tail and a powerful body.
Myrkas could create powerful electrical charges. These charges were powerful enough to kill a human, but the target needed to be close or touching metal which the Myrka was also touching.
Because they were created from a deep-sea creature, their skin withstood a lot of damage. They were highly bullet-resistant, and could take out a missile by themselves. They could also survive on land, but they moved slowly and had trouble staying upright. On the other hand, due to their origin, they were very susceptible to bright lights and UV rays. Small amounts stunned and potentially blinded them, but large amounts killed them outright. (PROSE: The Scales of Injustice, TV: Warriors of the Deep)
The Sixth Doctor encountered a Myrka while travelling with Evelyn Smythe in 1835. It was in the employ of the Silurians, and it was trained to specifically kill a single target. Due to this programming, it was able to be turned against its Silurian masters. (AUDIO: Bloodtide)
In the Pacific Ocean in 1886, a ship captained by Madame Vastra was attacked by a Myrka, causing her along with Jenny Flint, Rose Tyler and the Ninth Doctor to be shipwrecked on an island. The Myrka had been placed there within a perception filter by the Sixth Doctor to protect Horlak's tribe of Silurians. The Doctor noted the Myrka was doing this "a little too enthusiastically." (COMIC: The Lost Dimension)
In the 1970s, the Silurians used a Myrka as part of their invasion fleet on the Kent coast. Sergeant Benton killed it when he detonated a bag full of flares beneath its feet. (PROSE: The Scales of Injustice)
The Silurians used a Myrka on Earth, destroying probes Sea Base 4 sent out. They then used the beast to gain entry; it caused chaos, easily killing the human guards fighting it. The Fifth Doctor killed it through the use of ultra-violet light. (TV: Warriors of the Deep)
Upon drinking Aqua Galatia, the Fifth Doctor claimed that he felt so rejuvenated that he could run a marathon, wrestle a Myrka and then compete in every event of the Alpha Centauri Olympics. (AUDIO: A Requiem for the Doctor)