The definition of Black doom (also known as blackened doom metal) is a style that combines elements of black metal and doom metal. Typically, vocals are in the form of high-pitched shrieks and guitars are played with much distortion, which is common in black metal. But the music is played at a slow tempo with a much 'thicker' guitar sound, which is common in doom metal. Lyrics often involve themes of nature, nihilism and depression. The early work of Bethlehem (Germany) and Barathrum (Finland) is generally regarded as the foundation of this style. Pure blackened doom bands are fairly rare, but Dolorian (Finland), Unholy (Finland), Ajattara (Finland), Forgotten Tomb (Italy), Nortt (Denmark) and Gallhammer (Japan) have performed in this style.