Metal fans witnessed something Thursday night that many of them didn’t think would ever happen.

Yet, there it was — unfolding in all its thrash metal greatness — as mighty Slayer returned from a five-year retirement to perform a headlining set on Day 1 of the Aftershock Festival at Sacramento’s Discovery Park.

Talk about a feather in the cap for this four-day metal-a-thon, which continues through Sunday with such acts as Iron Maiden , Mötley Crüe, Tom Morello, Anthrax and Judas Priest still set to perform.

Slayer is, of course, one of the most important metal bands of all time — a member of the so-called “Big Four” of thrash metal, which also includes the Bay Area’s own Metallica as well as Anthrax and Megadeth.

And, prior to just a couple of weeks ago, the Huntington band hadn’t played a live gig since winding up their farewell tour at SoCal’s legendary Kia Forum on Nov. 30, 2019. (The group’s previous Northern California show fell on Nov. 26, 2019, at Oakland Arena).

Then, to the delight of their metalhead fans, they agreed to reunite for three shows — all of which were at festivals run by hard-rock/metal promoter specialist Danny Wimmer Presents. But only two of those three would end up happening — in Chicago on Sept. 22 and the Aftershock gig. The other intended performance, set for the Louder than Life festival in Kentucky, ended up being canceled due to weather issues.

So yeah, Aftershock fans were feeling pretty fortunate about being some of the select few to see Slayer in concert in 2024.

For its part, Slayer — which features the return of the 2013-2019 lineup with guitarists Kerry King and Gary Holt, vocalist-bassist Tom Araya and drummer Paul Bostaph — did its utmost to make this reunion live up to the hype, powering through a 20-song set of hard-hitting rock over the course of some 90 minutes.

Whatever rust might have formed during the break, the band apparently shook off (and then some) during the Chicago date. Because this version of Slayer was easily as good as what we got during that last Bay Area date in 2019.

The fans absolutely lapped it up, showing how much they had missed the band as they shouted out one profanity-laden message of encouragement after another to the musicians onstage.

Slayer then returned the fans’ passion (and cuss words) as it opened with “South of Heaven” — the title track to the band’s fourth studio album — and proceeded to touch upon 10 of its 12 albums during the solid show.

Araya and company drew most strongly from the band’s twin classics — 1986’s “Reign in Blood” and 1990’s “Seasons in the Abyss” — which collectively constituted a full half of the setlist. Highlights included “War Ensemble,” “Dead Skin Mask” and the “Seasons in the Abyss” title track.

For those who skipped opening day of Aftershock — or Slayer’s gig earlier this fall in Chicago — I wouldn’t have any major concerns about having missed out on all the band’s reunion fun. Although no further dates have been announced at this point, I fully expect that the group will keep the reunion party rolling into 2025. The band is just sounding too strong — and the demand for gigs is too great.

Although the Slayer reunion performance was the major selling point of Day 1 of Aftershock, it did not, however, turn out to be the best set of the festival’s first day. That distinction went to Halestorm — and how — as the Pride of Red Lion, Pennsylvania used its roughly hourlong set to once again illustrate why it’s the best act in all of hard rock right now.

On pretty much any day of the year, Lzzy Hale stands as one of the most powerful and mesmerizing front persons in rock ‘n’ roll history. Yet, toss in a special occasion and, well, fans really know they are in for something incredible.

“I want to thank you all for coming to my birthday party,” said the vocalist-guitarist, who indeed turned 41 on Oct. 10.

She’d celebrate her big day by leading her band through one thundering rocker after another, including such highlights as “I Miss the Misery” and “Freak Like Me.”

“Aftershock — where are my Sacramento freaks at?” Hale asked.

Oh, they were there in full force — right alongside the ones from Fairfield, Los Gatos and all other parts of Northern California, as well as Bakersfield and beyond, all of whom were digging the combined fretboard fireworks of Hale and the band’s equally amazing lead guitarist Joe Hottinger.

And that rhythm section is just off the charts, with Hale’s brother — Arejay — putting on a show for the ages back on the drumkit and Josh Smith keeping everybody honest on bass.

So many memorable moments in that set, but none more so than when Lzzy Hale tore right into a bit of Heart’s iconic “Crazy on You” during Halestorm’s own “I Get Off” — showing once again that she’s one of the few rock singers on the planet who can go toe-to-toe with the legendary Ann Wilson.

Other Day 1 highlights included Rammstein vocalist Till Lindemann making his U.S. solo debut in a dramatic all-red stage setting: Pantera nicely setting the table for headliners Slayer with its own offerings of metal classics and plenty of Zakk Wylde guitar greatness; and Insane Clown Posse drawing a huge crowd of both their dedicated “Juggalos” followers and curious onlookers.