When discussing mythology it is my belief that the story of Gilgamesh should always be brought up at some point, if for no reason other than how old it is. The Epic of Gilgamesh is considered to be one of the oldest pieces of writing in the world dating back to as early as 2150 B.C. It was recorded on a series of twelve tablets though most historians believe that it gradually was added to rather than having been written all at once as a single piece. The story chronicles the life of the fifth king of Uruk Gilgamesh and involves many elements of magic and divine intervention.
Birth of a hero
Gilgamesh was said to be born of the goddess Ninsun and the king Lugalbanda and was said to be born two thirds divine and one third man. He was said to be wise and strong as a meteorite and perfect. He also built the walls of Uruk said to be the greatest in history. The epic states that no man has or will ever match his greatness and was said to be a great protector of his people. He was however a very oppressive king known for forcing himself on women when they were wed and as such his people prayed to the gods for something to protect them from his wrath as no man could match him, hearing their pleas the gods created another man to be equal to him Enkidu. Enkidu was born into the wilderness as an adult and lived there for a time before word of him reached Gilgamesh and having had dreams saying this man would become his close friend and advisor had the Enkidu brought to his palace. From that point on it was said that Enkidu would restrain Gilgamesh whenever he would oppress the people.
The flood myth
The most well known part of the Epic of Gilgamesh is his interaction with the flood hero Utnapishtim. Gilgamesh sought out to find the hero after the death of Enkidu made him realise that one day he too would die so he sought a method of achieving immortality from Utnapishtim who had been made immortal by the gods. After finding Utnapishtim he is told that there is a flower at the bottom of the ocean that could make him immortal which he then set out to retrieve. He tied cement blocks to his legs to allow himself to reach the plant and upon picking it cut the blocks off and floated to the surface again. There he decided that he would test the flower first by having an old man try it and if it worked he would use the rest for himself. On his way back to Uruk Gilgamesh decided to stop by a spring and bathe and set the flower beside him. When he looked away a snake ate the flower though shedding it skin as it left having became immortal. After this Gilgamesh returned to his city in despair until he reached the walls he had built and realized that even if he could not live forever by virtue of his achievements he could be remembered forever a form of immortality itself.
Origin of the myth
Most scholars believe that Gilgamesh was in fact the name of one of the ancient kings of Uruk and did have the cities walls built. No record has been found supporting that his fathers name was Lugalbanda though. Other achievements in the epic are believed to have been done by other kings of the era and later attributed to him.
Modern Influences
Gilgamesh has became quite popular in modern media being referenced in everything from books to movies and even games. Some notable ones being his appearance in the fate series(foreground of the above picture), The Secrets of the Immortal Nicholas Flamel series, several operas titled Gilgamesh, and the Final Fantasy series of video games.
You can read a translated copy what has been recovered of the epic here.