Task: Question Answering
Input (English): Passage: The term "Viking metal" has sometimes been used as a nickname for the 1990s Norwegian black metal scene, which was "noisy, chaotic, and often augmented by sorrowful keyboard melodies". It has also been variously described as a subgenre of black metal, albeit one that abandoned black metal's Satanic imagery, "slow black metal" with influences from Nordic folk music, straddling black metal and folk metal almost equally, or running the gamut from "folk to black to death metal". Typically, Viking metal artists rely extensively on keyboards, which are often played at a "swift, galloping pace". These artists often add "local cultural flourishes" such as traditional instruments and ethnic melodies. It is similar to folk metal, and is sometimes categorized as such, but it uses folk instruments less extensively. For vocals, Viking metal incorporates both singing and the typical black metal screams and growls.
Overall, Viking metal is hard to define since, apart from certain elements like anthem-like choruses, it is not based entirely on musical features and overlaps with other metal genres, with origins in black and death metal Some bands, such as Unleashed and Amon Amarth, play death metal, but incorporate Viking themes and thus are labeled as part of the genre. Generally, Viking metal is defined more by its thematic material and imagery than musical qualities. Rather than being a mock-up of medieval music, "it is in the band names, album titles, artwork of album covers and, especially, in the song lyrics that Viking themes are so evident." Viking metal, and the closely related style pagan metal, is more of a term or "etiquette" than a musical style. Since they are defined chiefly by lyrical focus, any musical categorizations of these two styles is controversial. Thus, Viking metal is more of a cross-genre term than a descriptor of a certain sound. Ashby and Schofield write that "The term 'Viking metal' is one of many that falls within a complex web of genres and subgenres, the precise form of which is constantly shifting, as trends and fads emerge and fade." From its origins in black metal, Viking metal "has diversified (at least in aural terms), and now covers a range of styles that run the gamut between black metal and what one might justifiably term classic rock". 
Question: What genres are defined chiefly by lyrical focus?
Output (English): Viking metal.