Before neither earth, nor heaven, nor gods existed, there were two primordial beings: male Apsu and female Tiamat, representing fresh and sea water, respectively. Apsu has been a rather busy fellow and Tiamat soon gave birth to a large number of godlike beings known as Igigi. The most powerful among them were Lahmu and Lahamu, who engaged in Brother–Sister Incest to produce Ansar and Kisar, who followed in their parents' footsteps and produced Anu, whose son was Nudimmud and grandson, Ea (a.k.a. Enki). Since each generation became smarter, stronger, and more powerful than the last one, they ended up with a pretty Badass Family, collectively known as Annunaki.
After a while, Apsu got really pissed at Annunaki for making lots of noise and distracting him from work ("work" being, presumably, making even more kids with Tiamat, since nothing else existed at that point). Apsu consulted with his Igigi vizier Mummu and decided that killing the obnoxious progeny is the way to go, but Tiamat played Mama Bear and warned Ea. Before Apsu could attack, Ea put his great-great-great-grandfather to magical sleep, murdered him, and had Mummu Bound and Gagged. He then took Apsu's body and turned it into the hub for himself and other Annunaki. It was in Apsu that Marduk (Ashur in Assyrian version), son of Ea and his wife Damkina (presumably an Igigi), was born. Anu gave his great-grandson four winds for a birthday present, which the boy immediately used to annoy Tiamat. As the following would show, he had zero respect for his elders.
Meanwhile, the popular reaction among the common Igigi to the Annunaki's murder of Apsu forced Tiamat to take action against them. Keeping in mind what happened to Apsu, she carefully prepared an army of giant serpents (possibly dragons) and a Quirky Miniboss Squad of eleven lesser Eldritch Abominations (among them Hydra, Dragon, Hairy Hero, Great Demon, Savage Dog, and the Scorpion Man). She then re-married and appointed her new husband, Qingu, as the general of her army, giving him the Tablet of Destiny, which conferred to him the power over all gods.
When Ea heard about Tiamat ramping up for war, he ran to Ansar (de facto leader of the Annunaki at the time) for council. Ansar sent his grandson Nidimmud to vanquish Tiamat but he chickened out after seeing her and her army. The same befell Anu. Enraged, Ansar retreated to his tent, while Ea summoned Marduk and asked him whether he was a bad enough dude to take down his granny. Marduk, being the Marty Stu that he was, saw his chance and demanded full authority over all creation (then held by Ansar) to succeed. Ansar was reluctant and sent his vizier Kaka to his parents Lahmu and Lahamu. Faced with the prospect of extermination by Tiamat, Lahmu and Lahamu granted Marduk emergency powers, proclaiming him king of all Igigi. With his new resources, Marduk crafted a veritable Hyperspace Arsenal, consisting of a bow (his preferred weapon), a club, lightning and flames, the four winds that Anu gave him (which he supplemented with seven Evil Winds of his own), a chariot drawn by four Hellish Horses, as well as his signature weapon Thunderbolt (cf. Zeus). In the end, however, he was overcome with fear when he finally faced Tiamat's army, just like his grandfather and great-grandfather before him.
In the last moment, however, Marduk found his Heroic Resolve and confronted Tiamat about promoting Qingu, a literal nobody before the war, over Ansar, the born Top God, to her spouse. Unsuprisingly, his arrogance drove Tiamat into a bad fit of Unstoppable Rage, but before she could finish her spell, he immobilized her with his eleven Winds, then landed a One-Hit Kill on her with his bow. With Tiamat dead, her army attempted to flee but was surrounded and taken prisoner by the Annunaki forces. Marduk then took the Tablet of Destiny from Qingu and claimed it for himself, affirming his supremacy over the universe.
With the immediate threat gone and his power secure, Marduk had a bright idea: splitting Tiamat's vast corpse in two parts, he created heaven and earth. He then affixed stars to heaven (including the stars of Ea, Enlil, and Neberu) and created Moon (Nannar) and Sun (Shamash) to illuminate the place. He then formed the earth surface out of the other half of Tiamat's body (e.g. Tigris and Euphrates were made out of her eyes). Apsu was retrofitted as The Underworld, with Tiamat's eleven monsters placed to guard its gates. Lastly, Marduk established Babylon, "the Home of Great Gods" and his personal seat of power, before handing the Tablet over to Anu. Anu honored their earlier agreement and confirmed Marduk's kingship.
Soon thereafter, Marduk was visited by another nifty idea: instead of having to toil for themselves in this new creation, the gods should create a Servant Race of lesser beings called Lullu (or humans) to work for them while they were chilling out. On Ea's advice, Qingu was chosen to take the fall for Tiamat's rebellion and exsanguinated. Out of Qingu's blood, Ea shaped the first humans and tasked them with serving gods. Marduk, in the meantime, was busy assigning all 900 Igigi to their posts on heaven (300) and Netherworld (600). After all was said and done, the gods proclaimed him king once again, calling him by all 50 of his names. The last part probably doubled as a hymn to Marduk by the Babylonian priests.
