The old Mesopotamian gods were each patrons of a city, and their importance waxed and waned with the strength of that city. Babylon was, for centuries, the single largest city on all Earth. It was the pinnacle of human society's work, the centre of learning and culture nearly unparalled by all it surveyed. With this strength, so too did Marduk supplant Enlil and become known as the foremost amongst the Annunaki.
Marduk was said to be a temperamental god, quick to anger but willing to forgive. When King Sennacherib brought the might of the Assyrian empire down upon Babylon and pillaged the city, the wrath of Marduk soon found him: his son Arda-Mulissu murdered him. Sennacherib had not, however, acted out of simple hate or bloodlust; he was avenging another son of his. Marduk therefore did not curse all of Assyria nor all of Sennacherib's plans, as Sennacherib's chosen heir Esarhaddon - Arda-Mulissu's younger brother - prevailed in the conflict for the throne. Esarhaddon arranged for enormous restoration works to Babylon and its neighbours, an effort to make right the destruction wrought by his father, and even returned the stolen statue of Marduk. His reign would go on to be incredibly successful, and he even ensured a peaceful succession of his sons to both Assyria and Babylon despite the tumultuous nature of his accession. Marduk's wrath was great, but so was his capacity for mercy.
The end would come at the hands of an outsider. Babylon declined, but did not die, as Persia rose to dominance. Marduk remained respected and his rituals were observed, but the new imperial masters took their toll. Marduk was not dead, but he and his city were now shells of their former selves. It is perhaps ironic, then, then Marduk would share his fate with that empire. A conqueror from a backwater across the narrow strait to the west arrived, a young man by the name of Alexander whose armies fought with the all the might of Marduk himself when he struck down Tiamat. Alexander's march took him from Greece to Persia to the distant banks of the Indus, stopped at last only by the revolt of his own soldiers.
It wasn't long before Marduk's wrath found Alexander. The young conqueror was warned by Marduk's people that for him to march west, into the setting sun, would be the death of him. Even now, Marduk offered advice to his foe. They were right, and Alexander lay dead in Babylon at only 32 years old.
At last, though, Marduk could not forgive. His people had been crushed; his prophecy had been ignored; there was no heir of Alexander to heal the wounds. It was at this moment, stripped of his mercy and left only with wrath, that Marduk was no longer Marduk, and so he too died.
Babylon survived Marduk, but it would never again be the colossus that it once was. It would fade over time, its bricks stripped away to build new cities, until it was forgotten.
Marduk's death left us all a legacy that matched his temperament. Soon, humans identified and began to produce potassium nitrate, or nitre. This remarkable mineral is both life-bringing and life-ending, as it is a foundational component of fertilisers and gunpowder. Thus, even with his death, Marduk brought both wrath and mercy.
To be less flowery for a moment:
- The thing about patron gods is, in broad terms, thought to be true about the Mesopotamian gods. Marduk was basically a nobody until Babylon's rise to prominence, after which he replaced other gods in extremely important mythical roles due to Babylon's influence
- The stuff about Sennacherib destroying Babylon, his son murdering him, his two sons fighting over he throne, and the winning younger son rebuilding Babylon all happened. Babylonians at the time are said to have considered Sennacherib's fate a form of divine retribution
- There is some dispute over the character of Marduk, but we know of prayers praising both his fury and his mercy so I'm going with it
- The earliest mention of nitre that I know of is from 3rd century CE India, but the work in question really was (coincidentally) started roughly when Alexander the Great died. The finished work is a compilation of many other texts, and I do not know if the one mentioning nitre is one of the early ones