The story below comes from varying sources, but represents the story as found in the Enuma Elish. If you are familiar with my vlog, you'll know that I had already been speculating on the true identity of Slenderman, but I was wrong in my guess that he was a storm demon created by Marduk. No... he came before Marduk's battle with Tiamat as you will see.
When the heavens above were yet unnamed, and the name of the earth beneath was not yet recorded, there existed Apsu (the deep, also called Apason) and Tiamat (the sea, also called Tauthe or Tiawath). Their waters merged into a single boundless, confused and disordered mass of watery matter, and out from this mass were born, the gods of the earth and the gods of the stars. The gods of the earth, the Anunnaki, had the confused forms of animals, while the Igigi, the gods of the stars, were as man.
Many gods descended from the union of these two primordial beings. After long years, the god Anu was created in the image of his father, Anshar, and the god Anu begat Ea in the image of himself. Ea was endowed with understanding, a deep thinker and orator. He was unrivalled amongst the gods and attempted to establish a confraternity of the gods.
Here the tablets are damaged, but it appears Tiamat was troubled, “Her belly was stirred up to its uttermost depths.” The younger gods quarreled, and Apsu was distressed by the unrest. Tiamat gathered herself and struck a blow against the gods to bring them to order (al-ka-at, “the way“), but still Apsu was not satisfied. Then Apsu with Mummu (Moymis or Moumis), their son, visited Tiamat and took counsel concerning the gods. Apsu complained, "By day I find no peace, by night I have no rest. Verily I will make an end of their way, I will sweep them away. There shall be a sound of lamentation; lo, then we shall rest.” Hearing this, Tiamat was stirred to wrath against her husband. She took no part in the first struggle of Apsu and Mummu against the gods, as it was she who had in her charge and was guardian of all the strange creatures then existing.
They left the company of the goddess, and Apsu said to Mummu, "Whatsoever we have made we will destroy. Verily their way shall be filled with disaster; lo, then we shall rest.”
Mummu answered, giving counsel unto Apsu, "Come, do thou destroy their way which is strong. Then verily by day thou shalt find peace, and by night thou shalt have rest.”
They rejoiced at the thought of bringing the gods low and repeated their curses to their eldest sons. The gods made a lamentation, and Ea discerned their plan and brought it to naught. He made everything to stand still, and reciting an incantation, very powerful and holy, caused Apsu to fall asleep. He then loosed the joints of Mummu, who even so was yet strong enough to attack Ea when he turned to deal with Apsu. Ea overcame both his adversaries and used Apsu’s bones to build his palace. In one of the chambers of Apsu, Marduk was begotten and born. (He is sometimes called a son of Apsu.)
The rebel gods of the Earth, sons of Apsu and Tiamat, came to their mother and stirred her to anger over the death of her husband. They made plots without ceasing, exciting themselves to hostility. Tiamat called the stars and powers of the air to her aid. Ummu-Khubur (Mother Hubur, Tiamat herself or poss. a sister or daughter), spawned dragons, sharp of tooth and pitiless in attack, filling their bodies with venom for blood and decking them with brightness (ie she exalted them, made them divine). She created also the Viper and the Snake, The Whirlwind, Lakhamu, the ravening Dog, the Scorpion-man, The mighty Storm-wind, the Fish-man, and the horned Beast. (An exact parallel of this brood of devils is found in Egyptian mythology, the allies of Set and Apep referred to as the "Mesu betshet,” the spawn of impotent revolt, which are depicted as serpents. Some of them became the "Nine Worms of Amenti" mentioned in the Book of the Dead.) They carried the Weapon which spared not (lightning).
Tiamat created Kingu (Tammuz), her general and took him as consort, saying "I have uttered the incantation for thee. I have magnified thee in the assembly of the gods. I have filled thy hand with the sovereignty of the whole company of the gods. Mayest thou be magnified, thou who art my only spouse. May the Anunnaki make great thy renown over all of them." She gave him the Tablet of Destinies (Dup Shimati), fastening it on his brow, saying "As for thee, thy command shall not fall empty, whatsoever goeth forth from thy mouth shall be established."
Ea went to Tiamat to appease her but turned back in fear. Instead, he went to the dwelling of his father and Anshar. “Mother Tiamat who gave us birth hath sown these things. All the Anunnaki have joined themselves to her. They march by her side with those whom ye have created (some of the Igigi stood also with her apparently).”
Anshar bit his lips and said "Thou hast slain Apsu, but Tiamat hath exalted Kingu. Where is the one who can meet her?” And Anshar spoke to his son Anu, "Go and stand thou in the presence of Tiamat that her spirit be quieted, and her heart softened."
Anu took the straight road to her, but as he drew nigh, he saw Tiamat in her great anger and turned back. Returning to his father, he said, “She laid hands upon me that withered me up." Anshar was distressed, and all the gods shut their mouths and sat in lamentation.
Then Lord Anshar, the Father of the gods, said, "He whose strength is mighty shall be an avenger for us,” and appointed Marduk to go to Tiamat. Ea called his son to the place where he gave oracles and praised him. The Lord Marduk (Bel-Merodach) rejoiced at the words of his father and took up his place before Anshar.
Marduk said, "I will go, I will make true all that is in thy heart. O Lord of the gods, Overlord of the Great Gods. That I, your avenger, might slay Tiâmat and bestow life upon you, summon a meeting to proclaim and magnify my position. Sit ye down together in friendly fashion in Upshukkinaku and let me issue decrees by the opening of my mouth even as ye do (create). Whatsoever I bring to pass, let it remain unaltered."
And Anshar sent his envoy to the faithful gods, asking them to assemble at a feast. All the Igigi wept bitterly saying, "We cannot comprehend the work of Tiamat." They gathered together, and filling the court of Anshar, each gave a portion of his power to Marduk, becoming his fathers, and making him the equal of Tiamat.
He seated himself in the seat of kingship in the presence of the gods, who said unto him, "Thou art honorable by reason of thy greatness. Thy position is unrivalled, the words thou utterest become Anum (as fixed as the sky). The power to exalt to heaven and cast down to the earth both shall be yours. That which goeth forth from thy mouth shall be. Against thy utterance shall be no appeal. No one among the gods shall overstep thy boundary. O god Marduk, thou art our avenger. We have given unto thee sovereignty over the whole creation, and thy word shall be exalted. Thy weapon shall never falter; it shall break the head of thy foe."
To test his new power, a cloak was set in their midst. "Thou, Lord, shalt hold the foremost position among the gods. Decree thou the throwing down and the building up, and it shall come to pass. Speak but the word, and the cloak shall disappear. Speak again, and the cloak shall return uninjured.” Marduk spoke and it was so, and he spoke a second time, and the cloak reappeared. His fathers rejoiced, saying, "Marduk is King. Go, cut off the life of Tiamat. Let the wind carry her blood into the depths (under the earth, Magma?)."
Seven winds he then created to accompany him: four to take up their position so that no part of Tiamat might escape (The South, North, East, and West winds), a "foul" wind, the storm, the parching blast, and the typhoon, the wind incomparable. His final creation, the great weapon called Abubu, "the Flood," completed his arms. He dispatched the seven winds which he had made to make turbid the inward parts of Tiamat and raised up the wind storm, his chariot, the unequalled and terrible tempest. Marduk strung his bow and slung his spear. He raised the club and grasped it in his right hand. The bow and the quiver he hung at his side, and set the lightning in front of him. He held the net close to his side, a gift of his father Anu. He took a direct path to the middle (inside or womb) of Tiamat (a place, poss. a plain or caldera).
He sought out Kingu, Tiamat’s husband, and caused him to stagger, his will destroyed, his motion paralyzed. Tiâmat shrieked with lips full of rebellious words, chastising Marduk for coming in place of his father.
Marduk baited Tiamat, and she became like a mad thing, uttering shrill cries. The earth on which she stood split in twain at her words. The two great beings advanced to fight one other. The Lord cast his net, and the evil wind that had its place behind him, he let out in her face. Tiamat opened her mouth to cast a spell and the raging winds filled out her belly. Marduk grasped his spear and split up her belly. Her womb fell out from it and with it those creatures not yet born. He clave open her bowels and pierced her heart. He destroyed her life and cast down her carcass to stand upon it.
Her host was scattered, her levies fugitive. Her allies quaked with terror. They broke and ran but found themselves confined in the net. Marduk bound them and smashed their weapons. The Eleven Creatures which Tiamat had filled with awfulness were fettered, and the god Kingu who had been magnified over them was crushed and esteemed as little worth as Ugga (Dugga), a dead god. Marduk took the Tablet, pressed it with his own seal, and fastened it on his own breast.
After he had made captive the rebel gods, Marduk turned back to Tiamat and again trampled her body, clave in her skull, and slit open the channels of her blood, causing the North Wind to carry it away to a place underground. He removed her eyes, creating the Tigris and Euphrates. The Lord then paused and examined Tiamat's body. He slit her open like a fish cut into two pieces, separating hair from flesh (skinning her). The one half he raised up and shaded the heavens therewith, posting a guard that her water might not escape. He crossed heaven and betook himself to the abode of Ea. Measuring the dimensions of the Deep, he created the palace E-Sharra, which he placed over the bones of Apsu.
He appointed the Stations of the Igigi, the moon, sun, planets, and stars, and created man. But at the first sunrise, his creations died, not being able to bear the light of the sun. Marduk spoke unto Ea, "I will solidify blood, I will form bone. I will create man. Service to the gods shall be established and set the gods free."
Ea answered, "Let one brother god be given, let him suffer destruction that men may be fashioned. Let the great gods be assembled and let one be chosen from among them.”
A dispute broke out between Marduk and the gods, but Marduk said unto them, "Verily, that which I spake unto you aforetime was true. This time also I speak truth. Some there were who opposed me, who created strife and caused Tiamat to revolt. Let him who created the strife be given as sacrifice. I will cause the axe to do away his sin."
And the great gods, the Igigi, answered him, "It was Kingu who created the strife, who made Tiamat to revolt and join battle with thee." They bound him in fetters and brought him before Ea in the place of sacrifice, Uzu-mu-a, the bond of heaven and earth where Marduk locked the two halves of Tiamat into place. From his blood, the gods fashioned mankind for their service, and the gods were set free.
Though all the gods were said to have fashioned humanity from blood and clay, it was Marduk who was called tutu, creator, and he who was lord of the glorious incantation, bringing the dead to life. He riveted his judgment on the Anunnaki, his enemies, making heavy the yoke which he had laid on them. To redeem them, he created mankind and service was laid upon Man to do that work which pleased the gods not. It is through their service and worship of the gods and by their righteousness that they might be redeemed for their hostility.