Eris is the Greek goddess of chaos, strife and discord. She was the daughter of Zeus and Hera, although according to other myths, she was the daughter of Erebus and Nyx or just Nyx alone. Her opposite was Harmonia. The equivalent Roman goddesses of Eris and Harmonia were Discordia and Concordia. She is said to bore many children whom she brings along with her when she rode her chariot to war alongside Ares her brother. Eris is also the creator of the golden Apple of Discord.
All of Mt. Olympus and many gods and goddesses were invited to the wedding of Peleus and Thetis. Eris, however, was not invited. Angered, she arrived at the wedding in secret, and tossed a golden apple with the inscription "To The Fairest" on it. Most of the gods recoiled from this, and most goddesses as well. However, Athena, Hera, and Aphrodite did not recoil, believing they were the fairest, and argued over it.
Eventually, they consulted Zeus but the king of the gods did not want to get on any of their bad sides, so he chose a mortal named Paris to judge them. Hera offered him jurisdiction over a large piece of land, Athena offered him wisdom, and Aphrodite offered him the love of the most beautiful woman in the world. Paris chose Aphrodite and this caused a war between the gods known as the Trojan War. Later, he married Helen of Troy, which caused war between Troy and Greece.