The sumerian gods are a mystery to us but perhaps AI will offer insight..
Enlil has already declared that the lower gods shall be created through the blood of the earth. Enki asks for some time to think about the matter, whereupon Enlil declares that his decision is final and begins to destroy the netherworld. Enki and his allies, Marduk and Ea, then use their magic and power to defeat Enlil and stop him from destroying the netherworld, leaving only the upper gods (Anu and Nuhakhshah) behind. Enlil is so angered by this that he tries to destroy them, but Marduk is able to stop him with his magic again. Enlil curses the "chaos, darkness and death" that are upon the earth and he, Marduk and Ea leave.
But Marduk is still unhappy with the situation, as he feels that he is being oppressed by the gods. He is unhappy that Enlil is using him to take over the earth (which Enlil denies), so he decides to make an agreement with Enki, Ea and the other gods, allowing the gods to rule the earth from this time forward. Enki is glad to see his younger brother back again and both brothers swear their loyalty to each other. They then agree that both Marduk and Enlil shall have power over their respective regions (or cities) of the earth.
But Marduk is still unhappy and soon the gods plan a battle between Marduk and Enlil. Marduk wins and Enlil flees to the underworld, where he builds a place to live in with the other gods. He becomes the first king and all the other gods become his slaves. Marduk and Enlil, now King of the Netherworld, share the power over the underworld together. Enlil, however, is jealous of his younger brother and tries to trick him, saying that Marduk will soon attack him and force him to fight. Enki gives him some magic weapons so that Marduk can fight and kill his younger brother, but as soon as he does, he becomes weak and loses power. He goes back to the underworld, giving Marduk absolute power over his life.
Anu and the higher gods rejoice in Marduk's victory, as he will now rule them as king. They declare that the lower gods have no place in this new world, except for a few minor roles and Marduk agrees.
Anu declares that Enlil has now been banished to the Underworld, which they then go to meet him. There he learns that Marduk is his older brother and that Enlil is his younger brother. Anu is afraid of Marduk, for he has absolute power over the gods, so he agrees to help Enki in his plot against Marduk. Enki tells Enlil that he must create a place to live in the underworld for Anu and the other gods, so Enlil makes a place for them where Anu and the other gods live.
There is, however, no place for Enki to live, so Enki and the gods agree that he can stay wherever he wishes and that they will all be there to visit him, but not to interfere with his life in any way. They all agree that Enki will visit Enlil whenever they have a difference of opinion or that Enlil has any of his desires that he would like fulfilled. Enki declares that he will visit Marduk, but he will not interfere in his life. He will only go there if Marduk's desires interfere with Enki's plans.
In the underworld, Enki visits Enlil and tells him that Marduk has tricked him into making a place for Anu and the other gods to live, but Enlil is furious, since Marduk made him the slave of the underworld. Enki then reveals to Enlil that Marduk is in fact his brother and that Anu and the other gods are, in fact, the descendants of gods from an even earlier time.
This is is an interpretation based on the Myth of Eridu.
It's certainly a nice story, but it's quite a mishmash, so I don't think it's very authentic. You can't have Anu and the other gods ruling over the netherworld before they had their own separate existence. It is in fact not clear when the netherworld, which was also inhabited by the other gods, became the domain of the kings and then the gods, but I think that it's fairly obvious that it happened much later. It also doesn't explain why Enlil would willingly leave the netherworld and live in the upper world and give up all his power. He seems to have been very attached to the netherworld and to the gods, not the humans.
