Mythology inspires two kinds of creativity: retelling, and reinvention. The former is just taking a story we know and bringing it to life for a modern audience. The latter is taking the pieces of the story, or even other stories, and making something entirely new from them. Of the first we have countless examples in anime, many of such artistry and inspiration that those aspects alone make them special (e.g. Princess Kaguya). Arion is of the second kind; it reworks a grab bag of Greek myth into a new adventure, an epic of such ferocious energy and narrative fearlessness it beggars belief.
Against heaven itself
Little Arion was born, so it's said, to the goddess Persephone and the god Poseidon, in an illicit tryst that left the mother blinded by a curse and raising her son by herself. She's taught the boy all she knows about the use of healing plants, but she cannot protect him against the intrigues of the other gods. And so when Hades, lord of the underworld, come calling at her house with his three-headed dog Cerberus in tow, Arion's natural feistiness is of no protection. Down Arion goes into the underworld, where he somehow manages to hold his own against Hades's bestiary. Put a sword in his hand, Hades muses, and this kid's a champion.
What Hades has in mind is a war, an engineered conflict between Poseidon's dominion over the sea and Zeus's dominion over the land and air. Arion, all youthful pride and ambition, is a suitably naïve puppet to help speed such a conflict along. He believes Zeus is to blame for his mother's suffering, and all but spits in the eye of his commanders, Ares and Athena. The only one in their camp who shows him kindness is a mute servant girl, Resphoina, for whom Arion is instantly smitten. Arion also acquires a couple of sidekicks, a hulking three-eyed brute from the underworld, and a thieving brat who becomes, one could say, the Dororo to Arion's Hyakkimaru (and in more ways than one, as we find out in a surprise reveal).
When Ares's army pursues Arion to the ocean's edge, he falls into the hands of Poseidon's navy, who are only too happy to make him part of their war. But throughout all of this, Arion has only ever been a pawn of Hades, and his first blow for real freedom comes when he uses Hades's own weapons against him. It's a shocking moment, as it rams home for us how the gods of this story are not eternal, that they can and in fact are destined to die, and that the engine of their death may be not another god, but a man seeking freedom from the suzerainty of gods.
From there, the real arc of Arion's quest takes shape. He's no longer just a young man smitten with love, trying to rescue Resphoina from the lusty clutches of the likes of the harp-playing Apollon. He becomes the embodiment of human self-determination, of all humankind seeking freedom from the gods' machinations. For this he has the help of a great many other downtrodden human beings, but also one very specific one whose name and legacy came to emblemize such things in our world.
Yasuhiko's great myth
Two big advantages come to any story that uses a familiar mythology as a point of departure instead of just something to put a new skin on. One, as I've noted throughout, it gives you opportunities you didn't have with the original material. What makes Arion's quest so compelling is, again, how it constitutes as a struggle against the weight of history that is written in blood, not just one person's fight for what's his. The other advantage is how departing from the "official" version allows for more invention of all kinds. Arion's first two-thirds relies mainly on the tropes of fantasy warfare and swashbuckling; the third veers even into science fiction, as when Arion and his allies use a fantastic mirror-powered device that harvests and weaponizes the rays of the sun. Even better, it doesn't come off as an indulgence, but as another sign of how in this story's world humankind is making its own strides away from the gods.
Yoshikazu Yasuhiko, director and screenwriter of Arion (from his own manga), is best known for is iconic character design work on Mobile Suit Gundam and other shows. He has only directed a handful of feature films or TV shows; after the 1980s he concentrated chiefly on manga work and illustrations (The Dirty Pair's original covers and internal art were his work). Too often designers who get into the director's chair struggle to do more than just make things look good., but Arion is evidence Yasuhiko's storytelling chops were are refined as his design work., There's wall-to-wall spectacle in Arion, to be sure: the battle between Zeus's and Poseidon's armies features such mind-boggling sights as wolfmen diving on soldiers and tearing their throats open with knives clasped in their jaws. But it's all atop a story that demands our emotional investment and repays it bigtime.
One of my big critiques of anime as of late is that it has changed from "stories told with animation" or even "animated stories for younger audiences" to "stories for anime fans". Too much of it is about evoking known tropes or winking at its target audience, instead of using the power of the medium to create something wild and uninhibited and new. There are still great things being made today, to be sure (Beastars, Katanagatari, Dorohedoro), but I think the overriding lesson of the best old-school work like Arion was how it didn't hedge its bets about how to find an audience. It had no fears, just ambitions.
Normally I avoid hyperbole in my reviews, but here I grant myself an exception. Arion ranks easily as one of the best feature-film anime productions ever made. Everything beyond that is mere commentary. Seeing is believing.