Death in Venice is open to interpretation in terms of mythological elements in various ways. On one level, Aschenbach is a figure partaking of qualities of the Fisher King in Arthurian myth. In the most general sense (and probably most famously used this way by T. S. Eliot in The Wasteland) the King represents the one tasked with guarding some cherished symbol of the past. In the original story, it is the Holy Grail. But he is impaired, having received a crippling wound in the thigh or the groin.
Twentieth-century artists and intellectuals had begun to believe the Western world was entering a post-historical age, in which the great achievements had been completed and the culture had entered a period of decay and disintegration. In Death in Venice, Aschenbach is portrayed as an iconic figure in the arts, a writer whose works have expressed the noblest sentiments of his generation in an artistically polished and perfected mode. Aschenbach is thus a guardian of culture, as the Fisher King is, but he is also impaired, gripped by a disease—not a physical injury like the King's but a disease of the spirit. His obsession with the young boy Tadzio is not merely the shocking (in the context of the time) realization that Aschenbach is gay but is actually criminal, for Tadzio is underage and Aschenbach is potentially a pedophile. Aschenbach's artistry, his self-control, and his reputation as a cultural icon of his time are all in some sense a facade.
But it is not only a personal disintegration we see in Aschenbach. He is an emblem of the decay of European civilization. In Jungian theory Aschenbach's desire for Tadzio is the shadow, like the Freudian id the underlying desire that animates a person but is a hidden thing because it represents something unacceptable to society. But the desire can also be interpreted as the collective unconscious in which the entire civilization is somehow complicit. Aschenbach's nightmare reveals the landscape around his home in Germany and a savage orgy in which the populace is worshiping der fremde Gott—the alien, or stranger God. The veneer of civilization has fallen away and people (including Aschenbach) have reverted to their primitive ancestors.
This cataclysm is related to the prophecy of Norse and Germanic myth known as the Twilight of the Gods. Through the misdeeds of Odin (Wotan) the gods are destroyed. In Richard Wagner's re-think of this myth, the implication is that once the gods have been defeated, life must be seen in purely human terms. But in extending the idea behind this interpretation we can see that the laws, the order upon which the world rested, are gone—just as in Death in Venice the morality governing Aschenbach has fallen away like something outdated and obsolete. But Aschenbach dies on the beach, a victim of either the cholera epidemic sweeping Venice or the emotional shock of realizing that his whole life has been a lie. Just as Wotan and the gods are destroyed by Wotan's flaws, Aschenbach is destroyed by his flaw, and the cholera epidemic is analogous to the fire that destroys Valhalla at the end of The Twilight of the Gods.
Admittedly these interpretations involve numerous elements of myth which Mann may not have consciously intended as the subtext of his novella. However, it's precisely the unconscious way these elements are incorporated that is the hallmark of the deeper symbolism that animates great literature. Again, like Eliot's The Wasteland ten years later, Death in Venice can be seen as incorporating multiple elements of the desires and motifs that formed the basis of civilization's primal myths.