This ungodly gathering at the Post-Gazette Pavilion was the second annual Rockstar Energy Mayhem Tour, which functions these days as the successor to Ozzfest.

First, it was a sunbaked afternoon in the parking lot with thrashers like Behemoth and Cannibal Corpse pumping out machine-gun beats and monsterized vocals. The fans may have looked like a casting call for a "Road Warrior" sequel or a prison drama, but give them credit for having the discerning ears to tell all the bands apart and shout back lyrics buried in layers of distortion.

When the scene shifted to the main stage for the full-blown ritual, we were greeted by Mushroomhead sporting various white ghoul heads (perhaps because they were Browns fans playing in Pittsburgh -- ha!). 'Shroom is from the Slipknot school of scary metalcore with eight members bouncing around the stage, three of them beating drums. The band raged early, then oddly settled into near Linkin Park mode where songs like "Save Us" almost sounded like bids for alt-rock airplay. Somewhere in the chaos was a menacing version of Pink Floyd's "Empty Spaces."

The crowd was way more pumped for Killswitch Engage, a Massachusetts band that's worked its way up from second stages to the point where it has earned the right to pyro and more formal attire -- those fake tuxedo shirts -- with shorts. Led by belligerent frontman Howard Jones (not the synth-pop guy), the Killswitch nu metal approach involves coming on like jackhammers and then taking sudden melodic twists in the chorus. It seemed like everyone in the house was mouthing the words to songs such as "The End of Heartache" and "Holy Diver."

In what turned out to be peak craziness of the day, Jones engineered a "wall of death" on the lawn, which meant splitting the crowd for a "Braveheart"-like charge. It was a nice little icebreaker, a chance to make new friends and elbow them in the head before you trample them. The he-men kept that up for the rest of the set, and then returned from battle presumably to claim the available women, probably the ones who were pulling their halters down.

Slayer, the eldest members of the tour, stormed the stage proving its BPMs haven't dropped off over the past 20 years. It was the tightest musicianship of the day with more pile-driving rhythms from Dave Lombardo and the horrifying Kerry King, psycho guitar leads from Jeff Hanneman, and lyrical mayhem and head banging from Tom Araya.

With a stage of red light, fog, and Marshall amps stacked three high and a dozen across, Slayer unleashed songs of warfare, chemical warfare, terrorism, murder, suicide and, uh, facial scrubs ("Dead Skin Mask"). Satan got his due in "Hell Awaits," played on a stage ablaze with hellish fire and pentagrams. Somehow their fingers and necks held up for a relentless hour that only slowed down once, to go "South of Heaven."

It all built up to headliner Marilyn Manson, who delivered an anticlimactic set to fans who were pretty well spent by 10 p.m. Thirteen years since "Antichrist Superstar," Manson finds himself persevering after the shock has worn off and the industrial grind sounds so '90s. It didn't help that the fans aren't connecting to the new album, "The High End of Low," and its overly repetitive songs.

Even his shtick seems to have hit recessionary times, reduced mostly to confetti, a roadie that covers him with hats and flags and a lot of spitting of water or wine. Things perked up on older songs such as "Irrepressible Hate Anthem" and "The Dope Show," but clearly he was playing to a house that the previous band had already "slayed."