Doom is the original heavy metal. After all, it was Black Sabbath, on landmarks like 1971’s “Children of the Grave,” who invented its core traits: thunderously down-tuned guitars, lurching grooves and occult lyrics all cloaked in a funereal atmosphere reflective of the dread that had crept into hippiedom’s dying days. But while Candlemass and Saint Vitus, who helped establish doom as an identifiable genre in the ’80s, stick close to their heroes’ vision, behemoths like Melvins and Electric Wizard open it up to increasingly extreme dynamics, such as crawling tempos and raw feedback inspired by hardcore punk and noise rock.