The soldiers were lost time travellers converted into shadowy versions of their former selves. They included Earth humans, Gallifreyans and Tharils. They retained a humanoid form, appearing to wear gas masks and had wide-barrelled blasters attached to their right arms. The blasters could fire beams which could accelerate or reverse time in their target. If fired at living beings, they would be either aged to death or turned into infants.
Time Soldiers were unaffected by conventional weapons. Bullets and energy beams went completely through them. They could pass through solid walls or force fields. The only thing known to destroy them was their own time beams reflected back at them. Ace and Albion Strakk discovered that hylerium would do this. (PROSE: The Dimension Riders)