“Feud: Fire on the Mountain” more than earns its subtitle: The horror in the play erupts like hell on earth. To stage the legendary war between the Hatfields and the McCoys, Creighton James has written an honest-to-god tragedy, chockablock with anger, betrayal, and suffering. Such sprawling dramas have become rare in this one-act age, so it’s risky for an emerging writer to rattle the classical rafters. And for the most part, he rattles them hard.

Since it sets a tragic story in the Civil War South, the play shares similarities with Eugene O’Neill’s “Mourning Becomes Electra.” But where O’Neill adapted an existing Greek trilogy (“The Oresteia”), Mr. James is inspired by homegrown mythology. American audiences familiar with the Hatfields and McCoys, like Greeks who knew what happened to Oedipus, arrive prepared for the worst.

Mr. James’s script expertly tweaks those expectations. The story unfolds slowly, teasing us with catastrophe. For instance, when Nancy McCoy (Angelique Gray) runs to the door to meet her daddy, she finds her cousin Sam (Scott Price) instead. Sam was supposed to meet his uncle at the train station, but something has gone wrong. “I seen him!” the boy screams, clearly meaning the murdered Asa Harmon McCoy. But what did he see? The lights go black. We imagine the horror for ourselves.

Mr. James crafts every plot strand with the same assurance, showing all the ways that doom surrounds these families. A star-crossed love affair, a court battle, family infighting – each theme weaves a new thread into the bloody tapestry.

The excellent ensemble makes these moments chilling. All 18 actors are tightly controlled so that pivotal scenes feel devastating but never overwrought. Standouts include Keith Conway as Valentine McCoy, a vengeance seeking son, and Michael Batelli, whose good-ole-boy lawman offers crucial comic relief.

The actors’ in-the-moment intensity helps smooth the writing’s rough edges. Especially worrisome are the clear lines between good and evil. The story of the feud is just too grand for the shorthand by which Mr. James turns the McCoys into indigent saints and the Hatfields into black-hatted tyrants.

Still, Mr. James, who also directs, manages to mold the various Hatfield-McCoy episodes into a clear-eyed, gritty production. The slow burn of the early scenes lets the details sink in; clear stage pictures define who’s who. But as it progresses, the show dives into chaos. The actors get grimier, spitting food when they talk and drawing blood when they bite. Dirt covers everyone’s clothes, and the onstage musicians – a sort of bluegrass chorus – sing dark tales about the dead and dying. When the climactic violence arrives, the world we first encountered has exploded into a visceral nightmare.

***

The Fringe is the perfect testing ground for an ambitious play like “Feud.” But for every black-box gem in the festival, there’s some cubic zirconia like “The Mayor Who Would Be Sondheim.”

With what appear to be dreams of relevance, John Doble’s play tracks a politician who finds wisdom in the lyrics of Broadway musicals, and keeps singing Sondheim or Gershwin while his opponents argue about race relations and union rights. This bizarre conceit may be intended to make the mayor seem insistently idealistic. Instead, it just makes him – and the entire show – play like a cheap gimmick.

It doesn’t help that Mr. Doble can’t trust his audience to follow his arguments. A good 30 minutes could be shaved from the play if the mayor’s assistants would just stop reiterating the major plot points. And do we really need more aimless subplots about racist cops and sexist city officials? Simply mentioning political issues is not the same as exploring them. No matter how topical their subjects, plays this short on clear thinking become their own kind of tragedy.

“Feud: Fire on the Mountain” until August 27 (158 Bleecker Street, at Thompson, 212-279-2488).

“The Mayor Who Would Be Sondheim” until August 28 (197 E. Broadway, at Jefferson, 212-279-2488).