Stranger (The Coming of the Strangers)

The Strangers are an unnamed species of sapient invisible crustaceanoids from an unknown world, which invaded a British seaside town in the mid-twentieth century.

In their own language, they call themselves "men" and their homeworld "Earth", but their anatomy and psychology are both vastly different from Humans'.

Biology
Under normal circumstances, the Strangers are completely invisible, although they're still corporeal creatures which leave footprints and interact normally with the world around them.

They physically resemble Human-sized crustaceans, with six legs and strong pincers, which are sharp enough to decapitate a Human with relative ease. Their bodies have no pigments which reflect visible light; hence their invisibility. They are amphibious, and noted to be very slow on land, but agile underwater. Their bodies are enclosed in an armored shell strong enough to be effectively bulletproof.

The Strangers are silent creatures, which communicate solely by telepathy. They claim to have abandoned verbal language thousands of years ago. Their psychic abilities also allow them to subtly implant ideas into the minds of Humans, altering their behaviour and sometimes inducing them to become more aggressive; although their influence can be resisted by those who are aware of this.

Culture and society
In terms of psychology, the Strangers don't possess the same emotions as Humans, except for the basic emotion of fear, which is the only Human feeling that they can comprehend. It's been claimed that they don't care for each other and will abandon the bodies of their deceased.

Despite having razor-sharp chelae and bulletproof hides, the Strangers prefer to avoid physical confrontation and rely more on their invisibility and psychic manipulation to defeat their opponents. In physical combat, they will typically retreat. Despite being a spacefaring species with obvious access to technology, they carry no weapons with them, and tend to rely solely on stealth and indirect tactics. Still, their claims to have completely abandoned violence ten thousand years ago are obviously fabricated, given that they've been observed to kill Humans without provocation.

The only thing that the Strangers truly have in common with Humans is that both fear that which they can't comprehend and seek to eliminate threats preemptively: a feeling which triggered the Strangers' unprovoked attacks on Humans.

Appearances

 * The Coming of the Strangers, by John Lymington (1961)