The Capareth are a species of humanoids which hunt their prey by toying with their minds by creating illusions.
The Capareth appear as hunched human-like aliens, with black eyes, long talons and spines running down their back. They have incredible speed and accuracy, and can create illusions by using their prey's mind to fool their eyes. The Capareth are ruled by their Matriarch, which is not shown and commands her horde via a telekinetic link. This link can be used millions of miles away, which indicates the Capareth that attack the HMMS Aragard are commanded from the Capareth homeworld. The Capareth supposedly devour their prey entirely, and leave nothing but a skeletal remain. However they are not on the HMMS Aragard for simple scavenging, there is a powerful energy source that will feed them for millenia, the ultimate source of eternal life. The Doctor determines the energy is called The Source, and that it is a massive reserve of lifeforce. The Capareth are damaged by gunfire and other means of attack, but can easily heal by feeding again.
The Capareth prey on their enemies by fooling their eyes to imagine that their are things there that are not. They kill Pavada, an officer on the ship, by making him think there is a hole in the ship, and ambushing him when he is preoccupied. When the ret of the crew, along with the Doctor and Christina, find him, he is nothing more than a skeleton, drained by the Capareth, his "flesh torn from his bones" as they taunt. Captain Andari Tren, the captain of the HMMS Aragard leads his crew and the time travelers to the centre of ship, where beyond likes the Source. The survivors are mostly picked off by the Capareth, and Andari allows the Doctor and Christina to enter the location of the Source by sacrificing himself. The Capareth pack on the ship are destroyed when the Source is disintegrated by the Doctor.