There’s a new Ozzy Osbourne album out this month. There’s also a new Striborg album out: two and a half hours of oppressively bleak — and, at its best, downright chilling — “blackwave” courtesy of the Tasmanian loner known as Sin Nanna. I state these facts to demonstrate a simple point: The universe of what we call metal is vast, and more or less impossible to keep tabs on in its entirety. Favor the festival headliners and arena fillers, and you miss the best of the club acts and fledgling bands sweating it out over that soon-to-be-legendary demo; scour the underground obsessively and you might overlook a late-career triumph from a household name. So if this new incarnation of SPIN’s monthly metal roundup — in which I take over from Andy O’Connor, an astute and passionate lifer whose recom...