"I'm searching for Marduk, a creature the Babylonians worshipped as a god. But he doesn't live in Heaven, just on another plane."―Gennadi Volodnikov[src]
Marduk urged King Nebuchadnezzar of Babylon to construct a great engine to serve as a gateway between the two planes of existence: reality and the Aetherium. According to Soviet theoretical physicist Gennadi Volodnikov, "Marduk himself crossed over from the Aetherium to inspire the Babylonian priests"; however, during the completion of its construction, the Machine was suddenly abandoned by the uneducated masses, either by their own fearful act of defiance or because Nebuchadnezzar changed his orders. Regardless, four priests who may have remained loyal to Marduk named Urgon, Taklit, Azerim and Nub were able to escape with the four most crucial parts of the Machine and flee to opposite corners of the world (Kazakhstan, the Philippines, Mexico and Sudan respectively) wherein they each established themselves as a ruler among the local people and kept the parts safe within secluded sanctuaries.
Many centuries later, soldiers from the Soviet Union led by Dr. Volodnikov attempted to find these parts so that they could reactivate the machine and use the powers of Marduk and the Aetherium to take over the world themselves. Indiana Jones was able to find each of the parts before they did, but then made the mistake of giving them to CIA agents Sophia Hapgood and Simon Turner. Turner, corrupted by his newfound power, reactivated the Machine himself and pushed Sophia into a cage of the otherworldly substance aether, planning to send her down into the Aetherium for "scientific purposes." Jones killed Turner, but Hapgood was still sent into the Aetherium and Jones dove after her.
Within the Aetherium, Jones at last encountered Marduk, a bizarre, winged, quasi-physical hollowed being made of light and energy (presumably aether) with no apparent biological sex (but with the representation of a deep masculine voice that spoke fluent English, although it may have only sounded this way while he actually communicated with Jones telepathically; the only words Marduk ever spoke to Jones were, "Welcome to the Aetherium, mortal!").
The creature's power source came from a pink gem worn around his neck and known as the Marduk Medallion. This allowed Marduk to charge up his energy beneath a suspended electrical power sphere and consequently fire purple energy blasts. Upon encountering Jones, Marduk charged himself and then immediately commenced a violent barrage of attacks upon the human intruder. Jones, though, nearly defeated Marduk by reflecting the luminous attacks back at him with a large mirror; weakened, Marduk fled and later reappeared to graft his ghostly body with Hapgood's to form an even more extraordinary hybrid that resembled a giant aethereal harpy. This feminine half-Hapgood, half-Marduk monster, after killing Jones, would presumably leave the Aetherium to fulfill its original task of taking over the Earth. However, Jones was able to harness the same deadly powers that Marduk had used by charging his whip with the same aether energy. After striking the monster several times with lightning from the aethereally-charged whip, Jones sufficiently crippled Marduk's aethereal form, freeing Hapgood's body as a result. If Marduk survived beyond that, he was certainly in too weak a state of existence to still have any visible body left.
As the Aetherium collapsed and merged with reality, it is assumed that the already-diminished Marduk, unable to survive in a non-aethereal atmosphere, was erased, like the Aetherium itself, from all existence.