Tralfamadorians

The Tralfamadorians are a reappearing alien race that play a large role in the novel Slaughterhouse Five, and a smaller one in the novel Sirens of Titan. They are different beings in the novels.

Tralfamadorians in The Sirens of Titan
In the Sirens of Titan, the Tralfamadorians appear with the character Salo. He is a messenger from Tralfamadore whose spaceship malfunctioned and he was forced to take refuge on Titan, where he befriended Winston Niles Rumfoord. The book explains that the current Tralfamadorians are machines, built by the original Tralfamadorians. The organic Tralfamadorians wanted to know what the purpose of their lives were. When they eventually learned, most were disappointed with their purpose, which they believed was smaller than others. So instead, they built robots to fulfill their purpose, while the organics found better purposes. The organics continued to be disappointed by all their news tasks and built robots for all other purposes. Organics now had no purpose, and since they despised beings with no purpose, they tried to kill each other & themselves. They discovered that they were not very good at this as well, and assigned the task to robot Tralfamadorians. Eventually, all the organic Tralfamadorians were killed, leaving the machines to take their place.

The Tralfamadorians are very technologically advanced. Later, the book reveals that they have been manipulating earth's civilizations (using the universal will to become) to communicate with Salo and some landmarks on Earth, such as the Great Wall of China, are messages from Tralfamadore to Salo about the arrival of a necessary piece of machinery for his damaged spaceship.

Tralfamadorians from Slaughterhouse Five
In Slaughterhouse Five, the Tralfamadorians are four-dimensional, green, and plunger shaped. They are said to resemble a hand with an eye in the centre. They kidnap Billy Pilgrim and put him in a zoo on Tralfamadore. They can see through time, and can live in any moment of time. They are fatalists, as they know how the universe will end, but cannot do anything to prevent it. When they release Pilgrim, he goes on to explain to the people of Earth what he learned on Tralfamadore.