Sontarans

Militaristic aliens who look like short, stubby humanoid trolls.  Their home world Sontar, has a gravity superior to Earth's, which gave them superhuman strength.  The Sontarans were a cloned species (although they had once had a primary and secondary reproductivbe system), capable of hatching millions of warriors, and therefore of sustaining huge military losses in their millenia-long war with their galactic foes, the Rutans.  The Sontarans' weaknesses included a small aperture in the back of their neck called a Probic Vent, and the need to recharge their life force when outside their regular environment.  They were also vulnerable to coronic acid.

The Third Doctor first met the Sontarans in 12th-century Wessex when the stranded Sontaran, Linx, kidnapped scientists from the 20th-cenury to help him repair his starship.   Linx was shot in the Probic Vent by the archer Hal, and his ship exploded.

Then, circa AD 15,000, the Fourth Doctor, Sarah Jane Smith, and Harry Sullivan met another Sontaran Styre, who performed cruel experiments on captured humans to discover mankind's ability to resist a planned Sontaran invasion.  They defeated Styre who, drained of energy, perished.

The Sontarans used the Vardans to deactivate Gallifrey's transduction barriers and, under Stor's leadership, a Sontaran commando invaded the Capitol of the Time Lords.   They were defeated when the Fourth Doctor used the Demat Gun.

Finally, Sontaran Group Marshal Stike allied himself with the androgum Chessene and the scientist Dastari to obtain the secret of the Rassilon Imprimature, but was defeatedby the combined efforts of the Second and Sixth Doctor, as well as Chessene's betrayal.