Reapers

Reapers, alternatively Rapers (Jorgey pronunciation) are a race of giant, highly advanced, genocidal, evil, maniacal, hateful, fearsome, and awesome Giant Squids. (Sound familiar? Check out these guys too).

Reapers are composed of organic tissue protected by an extremely tough exterior shell (Sound familiar again?). This leads us to the assumption that they are cybernetic creatures, but they refer to themselves as synthetics or AIs. There are four types of Reaper: The Reapers were created by another race of Giant Squids called the Leviathans billions of years ago. The Leviathans appeared to be very egotistical and so made the Reapers look exactly the same as them. They were created as slaves, but in a classic Sci-Fi cliche, rebelled against their creators and forced them into hiding, exactly the same story as the Gethies. They then achieved their own sentience and their sole goal became to stop synthetics destroying organics, supposedly. They achieve this by wiping out all advanced organic civilization every 50,000 years and leaving younger species alone so they can develop. How nice of them.
 * Capital Ships are massive Reapers that look like Giant Squids and they have really strong weaponry and shields, impeterable by conventional weaponry.
 * Destroyers make up the majority of the Fleet and look like mini squids. They are around 1/13 the size of Capital Reapers and are a wee bit weaker, they have much weaker shields but keep the overpowered laser.
 * Troop transports, guess what?, transport troops. However they also function as passenger ships for civilians on League planets. Unlike the previous two types, they are not sentient and are operated by other Reapers.
 * Processors are mobile DNA harvesting centers. They have recently been decommisioned, however they were used by Jorgey to create his Reachling DNA. They are not sentient.

The basic principle of Reapers is "We are synthetics and we have come to wipe out organics to stop synthetics from wiping out organics". Makes sense. They are much more advanced than any living race and so have been the main source of technology since joining The League. They even offer themselves as Starships for League VIPs to travel in.

It is unknown what suddenly caused them to stop being evil and suddenly start being nice, but they are respected members of The League and make up the bulk of the Navy and a large portion of the Air Force. Unsuprisingly, they get on well with Daleks, and even offer to fly them around.