During Siege, Ares was killed by The Sentry. During Chaos War, since he's dead, he's enlisted into the Chaos King's army to destroy reality (and our Earth along with it, presumably). Seems like the sort of thing you can pretty much establish in a few panels, right? Well, get ready for four dollars worth of explanation, instead!
To be fair, Ares was always an interesting character who, under the right creative teams, grew beyond his Punisher-Meets-Thor pedigree. Oeming, however, isn't really given much room to play around with in this story. Ares is already dead, and in hell. The one-shot centers around the Chaos King invading hell, Ares trying to resist his influence, and then ultimately succumbing to it. In between, there's a lot of people, places, and things getting ripped apart. And that's about it. It's hard to really justify the price tag here, as nothing about Ares really changes here. He isn't actually resurrected into the Marvel Universe proper, and the little back story we do get about the character is really just fabricated to shed some light on his previous conflicts with the Chaos King.
The small army of pencillers and inkers also aren't doing the book many favors (they do, however, at least deserve to be credited by their full names, which they aren't). There is a flashback sequence, which I'm guessing is pencilled by "Rodriguez," as it is in a different style than Segovia's work throughout the rest of the book, but even Segovia's style is rendered fairly uneven by the phalanx of inkers dispatched to the book. He has a not-unpleasant Tom Raney feel to his art, but it is uneven in spots due to the inconsistencies.
While "Chaos War" is not entirely uninteresting in its main books, once you start venturing into tie-ins, like any crossover, quality starts to get dicey. "Chaos War: Ares" is not only a victim of what are undoubtedly deadline-based art inconsistencies, but also of relative insignificance. Ares is a good character when done well, but nothing in this issue signals any kind of major event in the life of the character, nor is it really essential reading for fans of the crossover. Thus, it's hard to justify the four dollar asking price, or even, really, why it was necessary that this story be told in the first place.