SOARThe self-titled debut album by System of a Down, released in 1998, revealed the quartet as a poised, powerful unit capable of expressing the personal and political with equal parts rage and wit, sometimes mashing them together in a zig-zagging whir of metal, punk and rap leavened by dashes of folk and jazz. Blessedly liberated from most metal clichés, the band demanded concentrated attention: At times, it was as if they had too many ideas for their own good.

That album, and aggressive touring to support it, paved the path for the successful 2001 follow-up, “Toxicity” – success that came in the form of worldwide sales in excess of 12 million copies, an astonishing figure in the era of Napster and other services that facilitated the theft of recorded music.

To newcomers, “Toxicity” revealed immediately the power of System of a Down. “Prison Song” banged along as metal rap until a shift in tempo drove the focus to singer Serj Tankian’s diatribe against the U.S. policies regarding incarceration, particularly for drug offenders. Hard-hitting drumming by John Dolmayan was matched by rapid riffing by guitarist Daron Malakian to tee up “Needles,” which also featured a tricky instrumental interlude, as did “Deer Dance,” a screed that painted an ugly picture of war between law enforcement and “peaceful loving youth.” “They like to push the weak around,” Tankian sang of the “battalions of riot police.” The songs were of a kind: explosive political rants that utilized alternations in arrangements to highlight a lyrical message.

The environment changed with “Chop Suey,” which opened with an acoustic guitar, jazzy chords on electric guitar, and hand drums; a misty bridge with textured vocals followed amped-up verses. The title track blended grunge and prog rock with metal, as Malakian played high-end arpeggiated chords to give the track an airy underpinning. “Psycho” rode in on Shavo Odadjian’s rumbling bass and in “ATWA,” the raging up-tempo bits served as interlude and the song unfolded slowly like a gathering threat. “Shimmy” found the band playing in the spaces between Tankian’s raps until Dolmayan and Odadjian offered a surprising touch of jazz, complete with walking bass. Roaring power chords on electric guitar and a shimmery notes on an acoustic counterpart helped form the foundation of “Aerials,” which featured Tankian’s most moving vocal.

The quartet share an Armenian heritage, and they invited the Armenian-American folk singer Arto Tunçboyacıyan to join in on “Forest,” which features tight, choked notes by Malakian. In his lyrics, Tankian revealed his belief that science fails to acknowledge the power of faith. “Spirit moves through all things,” he sang. “Science has failed our mother earth.” Tunçboyacıyan returned for “Arto,” the album closer, which was based on a hymn sung in Armenian Apostolic Church services.

With its political themes, “Toxicity” was met with controversy. After the 9/11 attacks, lines like “I don't think you trust in my self-righteous suicide” and “I cry when angels deserve to die” caused Clear Channel Radio to censure the track, though it was released a month before September 11. Nonetheless, the album sold more than a million copies in the U.S. in its first six weeks, and three singles from “Toxicity” – “Chop Suey,” “Aerials” and the title track – reached the top 10 on Billboard’s Modern Rock chart. Coincidentally, “Toxicity” was in the top slot of the Billboard 200 Albums chart that was released on September 11, 2001.

System of a Down thus far has released three studios albums in the wake of “Toxicity.” In 2006, they took an extended break; Tankian released a solo album, Malakian and Dolmayan formed the short-lived Scars on Broadway, and Odadjian worked with George Clinton and RZA. They reunited to perform in concert, but have yet to issue new material. On April 20, they will play in Moscow, followed by a concert on April 23 in Yerevan, Armenia, to commemorate the 100th anniversary of the slaughter of some 1.5 million Armenians by the Ottoman Empire. The latter will be their first show in the land of their ancestors. If previous 2015 shows are any indication, the concerts in Moscow and Yerevan will feature many songs from “Toxicity,” the band’s most successful album and a landmark in 21st century metal music.