Grievers are artificially engineered creatures that appear in the Maze Runner book trilogy by James Dashner and The Maze Runner film. They are ferocious, bulbous beasts with a somewhat arachnid appearance, travelling about on several mechanical legs with two organic feelers on the front of the body and a long mechanical tail on the back.
The Grievers seem to have been engineered by the WICKED organization as part of their Maze Trials wherein selected youngsters (Gladers) are placed within an artificial environment constructed in the form of a vast maze. The Grievers' purpose is to hunt down and kill the Gladers as part of the Trials, to test which ones are suitable for creating a cure for the Flare virus that threatens the human race.
Grievers can "sting" Gladers, or prick them, which causes extreme pain for up to days or weeks. Stung Gladers who take the Grief Serum are able to regain some of their memories during "the Changing." If a Glader does not take the serum, they die.