After this, […] present the Kyrie chant, whose liturgical function, at the beginning of the Mass, is to exalt divine majesty, the only force capable of remedying the weaknesses and imperfections of the human soul. Once a year, the Kyrie was sung outside the Mass, to open Vespers for Easter Day. […] But the Kyrie could also be employed to fortify the souls of the combatants when an army deployed in order of battle.
There are superficial reasons why metal compares so much to medieval music, to do with the lineage of the musical language it uses (metal started when Sabbath starting imitating horror movie music clichés – which themselves relied on a lot of ‘medievally’ sounding devices, because they sound ‘creepy’ to modern ears). More fundamentally, metal shares a dark but reverent worldview and mythic/allegoric way of describing life which is much more at home in the ancient world than it is modernity.