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 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)
Later, the Sixth Doctor encountered one while travelling with Evelyn Smythe in 1835. It too was in the employ of the Silurians, and it was trained to specifically kill a single target, but due to this programming it was able to be turned against its Silurian masters. (AUDIO: Bloodtide)