Lachesis was one of the three Fates, a goddess of destiny. She, along with her two sisters Clotho and Atropos, have the job of deciding the fate of everyone, including the immortal gods. Lachesis's specific job was to measure out the thread that Clotho wove, deciding how long a person's life should be and their destiny alongside it.
In mythology, it is said that she appears with her sisters within three days of a baby's birth to decide its fate. Her Roman equivalent was Decuma. According to Hesiod's Theogony, she and her sisters (Atropos and Clotho) are the daughters of Nyx (Night). In other accounts, they are said to be the daughters of Zeus and Themis as well as several other beings such as Chaos and Ananke.
Her symbol is said to be a scroll or a globe.
Lachesis is also mentioned in the tenth book of the Republic of Plato as the daughter of Necessity. She instructs the souls who are about to choose their next life, assign them lots, and presents them all of the kinds, human and animal, from which they may choose their next life.