Slithers are genetically-modified creatures designed by the Sathar as Attack Monsters.
A slither is 20 meters long and 2 meters wide. It resembles a cross between a worm and a centipede. It is part plant and part animal; plants grow from its back, providing camouflage and some nourishment from photosynthesis. It can only see moving objects, but can sense salt and water. One often lurks near pools of water or salt licks, waiting for prey.
When lying still, a Slither is often mistaken for a mound of plants, allowing it to attack with surprise. Its gnashing, grinding mandibles are surrounded by 2-meter-long tentacles. Slithers also secrete an oily fluid that causes burns on touch. They attack by lashing out with their tentacles, by biting with their jaws and by coiling around prey and burning it with their secretions. They must re-coil and make a new attack roll each turn to coil about prey. A slither can attack two creatures on the same turn, one with its tentacles and bite and the other by coiling around it. This makes them very deadly in combat.
Slithers are Sathar attack monsters. The Sathar, in their efforts to destroy peaceful worlds, have genetically altered a number of creatures into "monsters" which they set free on various worlds. Slithers and other Sathar attack monsters can be encountered on many different planets.
(Source: Alpha Dawn p.38)