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). They are led by King Squid, otherwise known as Harbinger.

Reapers are composed of organic tissue protected by an extremely tough exterior shell (Sound familiar again?) as well as extremely powerful shielding. 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 a being called The Catalyst. This catalyst was created by a race of giant squids called The Leviathans in an attempt to 'preserve organic life at all costs' due to the frequent number of synthetic rebellions during the time of the Leviathans. This Catalyst created an army of pawns to collect genetic data from species throughout the Galaxy and document them. Eventually, The Catalyst decided that the Leviathans themselves were interfering with its goal, and it decided to attack them. It used its armies of pawns to attack and kill its creators, collecting their genetic data and using it to build the first Reaper, Harbinger, who was designed after the Leviathans themselves. The remaining Leviathans went into hiding, shocked at the betrayal but believing that the Catalyst was doing its job, and leaving it alone. The pawns and Harbinger then went on to harvest the remaining life in the Galaxy and use the genetic data to create more Reapers. This cycle continued, with the Reapers returning every 50,000 years to harvest again. Obviously the cycle ended with the Reaper's defeat in the Reaper War.
 * Capital Ships are massive Reapers that look like Giant Squids and they have really strong weaponry and shields, impenetrable 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 everyone to stop synthetics from wiping out organics". Makes sense. They are 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.