Shelk

The Shelks are a sapient species of spider-like creatures from Venus with launched a devastating invasion of Earth in the year 3057, and ruled the planet for the next two millennia, pushing mankind underground.

Biology
Shelks are spider-like creatures, albeit with ten legs rather than eight, and stand about four feet (~ 1.2 meters) in height. Each leg has three joints and terminates in a short rudimentary claw. They have hairless reddish-hued skin, and their bodies are divided in two segments: a head and an abdomen, with the five pairs of long legs protruding from the mid-point between these two. The head and the abdomen are about the same size. Most strangely, their facial features are very reminiscent of a man's, albeit with neither hair nor nose. The eyes, mouth and general head shape are very similar to those of a Human, which only adds to their creepiness. They're carnivorous and breed other creatures as food, including Humans.

Having evolved under the perpetual daylight of Venus' sun-facing hemisphere, the Shelks have a strong biological aversion to darkness and dare not be active on Earth at night. They also have a strong collectivist instinct and will readily sacrifice themselves in battle to protect their leaders. This instinct of protecting the leader is so strong that wars against the Shelks have been compared to a large-scale game of chess in that once you capture the king, the game is over.

History
The first contact that the Shelks had with another intelligent species happened in the early 31st century, when a Human spaceship landed on their world. Prior to that, the Shelks were already an advanced species: more advanced than Humans in the fields of engineering, chemistry and many others. However, they were entirely ignorant of astronomy and unaware of the existence of worlds beyond their own, being not only isolated by the dense opaque atmosphere that covers their tidally-locked planet, but also ignorant of night, as they only inhabit the dayside of their homeworld.

Having found out that the Humans were not in fact divine creatures, but had come from another planet like theirs, the Shelks became suddenly aware of this new potential threat and acted immediately, taking advantage of their scientific knowledge to reverse-engineer the Humans' ship and mass-produce an entire fleet. With their knowledge of chemistry, they created weapons that humanity was completely unprepared for, and managed to take over the Earth, pushing the surviving Humans into an underground existence. For the next two millennia, the Shelks maintained control of the Earth. Eventually, mankind ceased to resist them and the idea that the Shelks cannot be resisted became ingrained in Human culture to the point that no one would even entertain the thought of fighting against them.

While some Humans lived in the underground corridors, others were domesticated by the Shelks and turned into different breeds, some bred for food, others to aid the Shelks in hunting, like the Human equivalent of a greyhound.

Appearances

 * "Tumithak of the Corridors", by Charles R. Tanner (1932)
 * "Tumithak in Shawm", by Charles R. Tanner (1933)
 * "Tumithak of the Towers of Fire", by Charles R. Tanner (1941)
 * "Tumithak and the Ancient Word", by Charles R. Tanner (2005)