|The information in this article comes from Mars: The Home Front and is not written by Edgar Rice Burroughs.|
The Sarmaks are one of the races that inhabited Barsoom. They are one of the few Martian races that humans are aware of, mostly because they invaded Earth in 1898 (as described by H.G. Wells in his story The War of the Worlds).
The Sarmaks are being described as a leathery-skinned, tentacled race. They have no digestive organs of their own, and thus are forced to feed on the blood of creatures that do have digestive organs. They feed on both Martian animals and Red Martians themselves. A journalist witnessing their landing on Earth described them as octopus-like creatures. They reproduce asexually, children being born by "budding" off a parent. Internally, the Sarmaks consist of a brain, lungs, heart, and blood vessels.
Despite having only primitive bodies, the Sarmaks are arguably very intelligent, being able of constructing advanced weapons and war machines like the Heat Rays and the Tripods.
Their origin and how long they have been living on Barsoom are unknown. According to the Wold Newton Universe, the Kaldanes are mutated Sarmaks, and both species are descendants from the Cthulhuhoids. In "Mars: The Home Front" Bas-Ok also compares the Sarmaks to the Kaldanes, but states that despite their common goals of becomming just the perfect brain, the Kaldanes are vastly inferior to the Sarmaks.
Whatever their origin, the Sarmaks had managed to stay undiscovered on Barsoom for a long time, possibly by living underground. All of that changed when they started their plans for the invasion of Earth. To this end they constructed a huge device capable of fireing cylinders containing both Sarmaks and material for war machines to Earth. They had multiple Red Martians working for them to carry out this plan.
John Carter discovered the location of this cannon (consisting of a giant pit in the ground) when Dejah Thoris was captured by the Sarmaks. After discovering that his native planet was in danger, John Carter fought the Sarmaks and managed to destroy the cylinder-launching device after the tenth blast, effectively putting an end to their plans. He then rallied the forces of Helium and all its allies (including the First Born and the Tharks) to destroy the remaining Sarmaks on Barsoom.
The few Sarmaks that did make it to Earth did not succeed in their plan of conquest. Although they initially managed to conquer the United Kingdom, they eventually died because of Earthly bacteria. By the time John Carter visits Earth and tells his nephew about his encounter with the Sarmaks, the invasion is already over.
The Sarmak are an adapted version of H.G. Wells martian invaders. The Sarmak are one of three examples of the invaders being adapted into an unofficial crossover with Edgar Rice Burroughs martian tales, the others being in the book The Alternate Martians, and in the League of Extrodinary Gentlemen, Vol. 2, where they are simply called 'the Molluscs'.