So here is what I understand beggining tiamat absu created heaven earth and all the gods Anu EA Bel and the rest the gods become Evil Tiamat wanted to destroy them the Gods brought up their Wise Counseler Marduck gave him power he managed to kill tiamat and is Ruler of the World Further Creating the Universe as He was granted more power. He is said to be a Wise and Loving God-- the interesting thing is why Tiamat wanted to kill her children for being Evil? And Tiamat s legions are scattered and slain by the elder gods and btw the necronomicon is real`
They didn’t exactly become “evil” more like they were being rowdy and Abzu’s advizor nammu or mummu suggested they be killed. It’s kind of the usual creation myth of the primordial beings being put to rest by the younger Gods, depicting a change in eras.
I can’t comment on the necrocomicon part because that’s fantasy to me.
There are alot of points that make necro reliable in my eyes first the names of the gods are the same second the story goes somewhat in persia summer translated from arabic which is another point all the goda inside including their calling exist. The 50names of marduck are mentioned in the tablets of creation I belive because it’s powerfull it ran out in to the mases they coudnt stop it so organizations tried discrediting the work to keep people away from it.
I believe @DarkestKnight explained a bit about the necronomicon, how it’s fiction but still works for whoever chooses to use it.