Fates

The Fates (in Greek Μοῖραι, Moirae), also known as the "apportioners," are the three white-robed personifications of destiny in Greek mythology. Their Roman counterparts are Parcae or Fata. The Greek word Moira literally means part or portion and by extension one's portion in life or destiny. They controlled the metamorphical thread of life for every living thing from birth to death.

History
The Fates are given several origins, most commonly called the daughters of Nyx. They control the fate of every being in the universe and weave the lives of mortals into the Threads of Life. Their power is very great and mysterious.

The Fates are extremely unpredictable, but seem to be at least somewhat benevolent as they appear after Luke's heroic death to give him a glorious send off.

Clotho spins the Thread, Lachesis decides how long a life is to be, and Atropos cuts the lifeline with her shears. All three decide on the destiny of all things.

During the Gigantomachy, the Fates joined Heracles and the gods in fighting the Giants. They told the Gods the secret on how to defeat the Giants (they have to join forces with Demigods to fight them) showing that the Fates were on the side of the Gods (for now).

The Lightning Thief
When their school year ended, Percy and Grover were taking a bus from Yancy Academy back to Manhattan, the bus broke down on a stretch of country road. On the other side of the highway was an old fashioned fruit stand, occupied by the Three Fates, as three old ladies sitting in rocking chairs, knitting a huge pair of socks using an electric blue yarn. The lady on the right and left were each knitting a sock, while the middle one was holding the yarn. Percy described them as ancient, with their silver hair tied back in a white bandanna, wearing a cotton dress and having bony arms.

They were looking right at Percy when the middle one took out her scissors, a large, gold and silver long bladed one similar to shears, and cut the yarn. The resulting snip could be heard over the four lanes of traffic. They balled up their yarn afterwards. Grover, seeing this, was extremely scared, thinking the yarn was Percy's lifeline, and that he was going to die soon.

The Titan's Curse
The Fates were mentioned by Artemis while Zoë Nightshade was dying, Percy asked the goddess why she could not heal Zoë of her wounds. Artemis replied that life is a fragile thing and if the Fates will the string to be cut, there is little she can do but try. Percy also says during the council of the gods that "controlling the prophecies never works" because prophecies are what the Fates have decreed.

The Battle of the Labyrinth
Right before Percy is about to decide if he will stay with Calypso or go back to Camp Half-Blood, Calypso explains the nature of her punishment. The Fates every few hundred years send her someone that can never stay for long and will always be someone that she can't help falling in love with. She claims the Fates are cruel as they always send someone that would break her heart in the end. Right after she offers Percy the chance to stay, she says that she wasn't going to but did it anyway, something that the Fates may have known she would always do.

The Last Olympian
The Three Fates appeared on Olympus to take away Luke's body after the end of the Second Olympian War. One of them looked at Percy and he saw his life flash by. She said "it is done" holding up the snippet of blue yarn, Percy knew instantly that it was the same blue lifeline that he had saw them snip four years ago. Percy first thought that it was his lifeline, but then realizes that it was Luke's, the life that had to be sacrificed to set things right. After Luke's body was taken away, Hermes says that "no one can tamper with fate, not even a god."