Alien (The 27th Day)

This unnamed humanoid species sought refuge from the death of their homeworld, and tested Earth's qualities by giving humans the ultimate superweapon.

Biology
The aliens appear externally indiscernable from Humans, and Earth's environment is hospitable to them, ideal for them to colonize. Any differences in physiology are unknown.

Culture and society
The aliens are pacifists, refusing to turn any weapon against an intelligent species, to the point that they are willing to let themselves die rather than directly hurt another. However, when testing the character of another species, they are willing to provide that species with weapons of mass destruction, seeing whether or not said species is willing to use them for conquest. To properly test a species, they select members of the general population at random, not necessarily anyone in power or of importance. They will not intervene throughout the testing process. Alien attire is known to consist of reflective, dark, long-sleeved jackets with matching pants and boots.

Technology
The aliens have highly advanced technology, capable of flying through space at lightspeed with saucer-shaped spacecraft. They have also developed universal translators, allowing their speech to be heard in the native language of whoever listens to them.

Appearances

 * "The 27th Day", by John Mantley (1956)
 * The 27th Day (1957)