Medusan

The Medusae (singular Medusa or Medusan) are an ancient and malevolent race of jellyfish-like aliens from a planet orbiting Barnard's Star. They are in need of iron, living space, and other resources, and attack Earth for this purpose.

Description
Medusae are flying aliens that vaguely resemble jellyfish, though in truth they are like nothing in the Solar System. A Medusan's main body is a translucent green hemisphere about twenty feet across. Thin but strong black tentacles descend from the hemisphere's rim to a length of fifty feet. Four purple eyes, each about a foot across, ring the hemisphere. They are spaced equidistant from each other, giving the alien all-around vision.

Medusae do not fly by way of wings or natural buoyancy. Instead, they levitate using a three-foot-wide glowing organ on their undersides that lifts them using an unknown radiant force.

The Medusae cannot speak and are totally deaf. Instead, they communicate using natural shortwave radio. When heard on human radio systems, this radio speech sounds like an unnatural hissing, crackling noise, though humans can be taught to understand it.

Homeworld
The Medusae home planet is an unnamed planet that orbits Barnard's Star, a red dwarf six light-years from Earth. The planet occupies a close orbit, but is not close enough to be tidally locked, as it has a rotation period of fifteen days. Save for its six moons, it is alone around Barnard's Star, which has no other planets.

The planet is an enormous terrestrial world about three times the diameter of Earth, but with similar gravity. There is a single large continent, with roughly the surface area of the entire Earth, while the other eight-ninths of the surface is taken up by ocean. The continent's terrain is dense jungle near the coasts, with mountains inland and an arid plateau at the center. The air is oxygen-rich and would be breathable to humans if not for an artificial red gas the Medusae use for climate control. This gas, which colors the sky red and the sea yellow, is poisonous to humans in the long term, causing madness and vaguely gangrene-like symptoms over a widely variable length of time. The week-long nights are extremely cold, hazardous even to natives and lethal to people, though the red gas mitigates this somewhat.

The planet is far older than Earth, and its life has had longer to evolve in a much harsher environment, making native organisms much more dangerous than Earthly life. However, the planet's long life is coming to a close - its orbit will soon (in relative terms) decay and send it falling into Barnard's Star.

Society and Culture
The Medusae are a cold, emotionless race utterly devoid of morality. What they want or need, they take. After depleting the resources of their homeworld billions of years ago, they set about raiding other planets for resources (especially iron) and eventually began setting up colonies on suitable planets. Any civilizations inhabiting these planets were exterminated. Any alliances they make are ruses to gain access to new worlds.

The Medusae live in vast cities exclusively designed for flying creatures. There are no streets, simply chasms between buildings, and doors frequently open into empty air at dizzying heights. The buildings, which are arranged with no apparent rhyme or reason, can reach heights of up to 10,000 feet. Everything is made of the same iron-free black alloy that makes up most Medusae artifacts. Only one such city exists on the Medusae homeworld at present, though there were more in the past, and others presumably exist on Medusae colonies.

Technology
The Medusae are an ancient and highly advanced species. They are capable of interstellar faster-than-light travel. Their spacecraft resemble great black metal spiders, with a central sphere 1,000 feet across and struts spanning half a mile - far larger than any human spacecraft they encountered.

The Medusae are capable of engineering on at least a planetary scale. They have set up their planet's six moons to project a force field called the "Belt of Peril," which dissolves the molecular bonds of attacking ships and their crews alike. They have stabilized their planet's climate using an artificial red gas of organic composition, which saturates the entire world's air and water. This gas is harmless to native life, but causes physical and mental decay in Earthly life at unpredictable rates. The Medusae are well aware of the gas' deadly effects on humans and frequently use it as a weapon. Finally, the Medusae set their entire star in unusually rapid motion relative to the rest of the galaxy, allowing them to raid the many planets they encountered. However, they are unable to prevent their planet from ultimately falling into Barnard's Star.

Iron is a rare but essential ingredient of Medusae technology. They have used up their planet's entire supply of iron, forcing them to find it elsewhere. Consequently, the Medusae build using an iron-free black alloy of unknown composition.