Sontarans are an evil race of warriors from the planet Sontar. For years they have been at was with their enimies the Rutans, both Sontarans and Rutans clone themselves to have massive armies in order to defeat each other. Sontarans love war and battle, they even invading the home planet of the Time Lords, the Doctor had encountered them many times and so did his old friend Sarah Jane. When the Daleks and Time Lords went to war with each other, which became known as the Time War. The Sontarans asked to join them in war, but both sides disagreed.
The Sontarans were also part of the alliance to trap the Doctor in order to stop the cracks in time.