Only Swans could bury two mantric, 30-minute squalls at the hearts of their most accessible records. (Accessibility is, of course, highly relative.) In the amount of time it takes Michael Gira and cult-mates to slay the beastly "Bring the Sun/Toussaint L'Ouverture" — track four on the band's second straight double-LP monstrosity, To Be Kind (Young God Records) — you could watch an entire boxing match or listen to the whole new Sleater-Kinney album; you'd emerge with far fewer bruises, too. The power Gira wields with this reconstituted 33-year juggernaut never was in question, but just how long a listener can sustain it before ripping off the headphones or running from the room very much is. This depends on many things: physical endurance, mental stability, intestinal fortitude, masochism. On 2012's The Seer and last year's fraternal follow-up, Gira introduces another factor: compulsion. Combined, these mirror-image behemoths unload 21 tracks and more than four hours of abusive sound that lands like chain weights directly on the shoulders. It's a difficult slog even for devotees, and so Gira loads it with an almost unbearable forward momentum and a breathable internal sequencing that becomes, after so much time, something like 110-decibel meditation. He's both Gregorian enchanter and Styx riverboat captain, a metallurgic industrialist and downhill Sisyphus. Little Annie and Paul Wallfisch open. Tickets $20 in advance, $25 at the door.