The Vigil were responsible for the sacrifice of the Queen of Years, should she choose not to sacrifice herself.
History
Clara Oswald first encountered the Vigil when they were hiding in a storeroom. They were looking for Merry, who had run away due to her being scared. The same three Vigil attempted to fulfil their duties when the Eleventh Doctor and Clara Oswald protected the then-current Queen of Years, but disappeared after they had failed, as the Old God had no further need of them.
Abilities
The Vigil had very soft voices, and were terrifying to most people who encountered them. In battle, they had powerful telekinetic powers which could be used to hurl their victims or shield themselves from attack. However, even when all three of them were attacking at once, they were unable to overcome the Doctor, who defended himself with his Sonic screwdriver. Whenever they used their telekinetic powers they were heard snarling and the Sonic Screwdriver's ability to deflect them meant that they must have been using sound waves caused by the snarls.
The Vigil also possessed the ability to teleport, which appeared to be their most common form of travel. (TV: The Rings of Akhaten)