Apple of Discord.

 Apple of Discord is also known as Golden Apple of Discord (μῆλον τῆς Ἔριδος). It created a dispute between three goddesses.

Eris wasn't invited to the wedding of Peleus and Thetis. She became angry.

So, she decided to give them the Apple of Discord. Eris wanted to make the goddesses quarrel. 

Athena, Aphrodite, and Hera saw the Golden Apple of Discord. It was written "to the most beautiful" (τῇ καλλίστῃ). The goddesses began to argue about it.

Paris judged Athena, Aphrodite, and Hera. He gave the Apple of Discord. She helped him steal Helen of Troy. Later the Trojan War started.

This legend became famous in Ancient Greece. Also, the phrase "apple of discord" means that small quarrels can turn into big conflicts.

Thetis.

Thetis (Θέτις) was a nymph and a goddess in Greek mythology. Her parents were Nereus and Doris. Or maybe she was a daughter of centaur Chiorn.

She was known as the wife of Peleus and mother of Achilles. Peleus and Thetis had a great wedding. Later the Trojan War started.

Thetis wanted to protect her son. She dipped Achilles in river Styx. But Thetis forgot about his heel. She held his heel.

Also, Thetis had relationships with other gods and goddesses. Themis, Dionysus, Hera, centaur Chiron, and Zeus. Some of them predicted early death of Achilles.

During the Trojan War Achilles was killed by Paris. 

This legend became popular in mythology.

There are many paintings and statues about this goddess.

Source of image:

https://www.shutterstock.com/ru/image-photo/large-set-ancient-greek-vases-plates-565660501

Source of image:

https://www.shutterstock.com/ru/image-vector/orange-black-figures-pottery-amphora-painting-1271788915

Source of image:

https://www.shutterstock.com/ru/image-photo/statue-nymph-thetis-trieste-italy-12441583

Eris.

Eris (Ἔριδα) was a goddess of discord and strife in Ancient Greece. Her parents were Zeus and Nyx or Erebus and Hera. Also, she had many siblings among other gods and goddesses. Aphrodite, Hermes, Ares, Athena, Heracles, and ect. were all her relatives.

Eris had her own symbol. She was known for her Golden Apple of Discord. Eris gave her symbol during Peleus and Thetis wedding. So, Athena, Aphrodite, and Hera became angry. After this event the Trojan War started.

Eris had several children.

There are many paintings and statues of this goddess.

Peleus.

Peleus (Πηλεύς) was a son of Aeacus and Endeïs in Greek mythology. Also, he was known as father of Achilles (Achilles' heel).

Peleus had relationships with other gods and goddesses. Including Zeus and Heracles. His brother was Telamon. Peleus was a hero and king of Phthia and the Myrmidons. He fought with his enemies.

He married Antigene first time. But she died.

Peleus married Thetis. There were many guests. Gods and goddesses had a feast. But goddess Eris wasn't invited on this wedding. She became evil.

During this event Hera, Athena, and Aphrodite had a dispute. The Apple of Discord made everyone quarrel. Eris gave them the Apple of Discord. Afterwards, goddesses had a Judgement of Paris. Later the Trojan War began.

There are many myths and legends about this hero. Artists created paintings.