Chicago's Nachtmystium have been haunting the dank basements and bat-filled belfries of America's burgeoning black metal scene since 1998, but like most of their countrymen, their works were generally too indebted to Scandinavian sources to earn them credentials to contend on the worldwide stage.

That may be about to change with 2006's Instinct: Decay, an album that generally stays true to the group's intentionally primal and lo-fi black metal roots, while simultaneously plunging over the abyss of progressive experimentation.

Virtually all of its tracks are fueled by the same sort of questing fearlessness that extreme metal fans have come to expect from contemporary artists like Norway's Enslaved and Sweden's Opeth: uncompromising, heavy as f*ck, but streaked by multiple moods, dynamic density, and unexpected twists.

Following the brief introductory "Instinct," "A Seed for Suffering" launches the album on traditionally pounding, raging, and desolate black metal brutality; yet it suddenly cracks open like an egg to reveal an evocative acoustic bridge worthy of Portland folk-metal stars Agalloch, then files a coda of renewed violence with melodic nuances that impart an emotion rarely heard in black metal: hope! Subsequent notable cuts include "Chosen by No One," which alternates frenetic riffing with groovier sections also streaked with near-symphonic layers of synthesizer melodies; "Eternal Ground," which includes a memorably musical guitar solo and ending riffs more blues-based rock than proper metal (a trick Darkthrone have flirted with in recent years); and "Here's to Hoping," which goes folk-metal in an entirely different way thanks to steady-thumping polka rhythms that could turn slam dancing into square dancing.

Maybe not...

Anyway, those with a thirst for more conventional, unadorned black metal carnage can still get their kicks from traditionally thrashy, buzzsaw-riff-and-blastbeat fests like "Antichrist Messiah," "Abstract Nihilism," and "Keep Them Open" -- though the third, too, offers an unforeseen element with its garbled computer noise outro.

Ending spacy and atmospheric instrumental "Decay" puts the lid on a masterful effort: at once retro and avant-garde but, most importantly, well composed and entertaining from start to finish.