Lachesis is one of the three Moirae, or Fates, of Greek mythology. Her task is to measure out the thread of a mortal's life, just as Clotho's job is to spin it and Atropos's is to cut it.
The triple-goddess is a common archetype in Greek myth. The Furies, the Graiae, or Grey Sisters, and the Gorgons are other good examples of the Three-as-One. It's thought that these goddesses actually belonged to an earlier, chthonic, possibly matriarchal culture and were adopted/adapted by the Acheans to suit their own purposes. Not entirely impossible, or even improbable. Look at Dionysus for a good example of cultural blending.
The exact origin of the Fates is unknown. They are said to be more powerful than even the gods in their own way, and their work, once done, cannot be undone. (See the story of Alcestis and Admetus for more information on that.)
Side note: there are Norse counterparts to the Fates of Greek mythology-- the Norns.
Lachesis’ “double” is Verdandi, who winds the thread of fate. The “job description” for both sets of deities is practically the same, making one wonder about the truth of a supposed collective unconscious.