Horace Rosenqvist first started composing the music of Aquilus in 2000, in Melbourne, Australia. The music is a blend of atmospheric black metal with symphonic arrangements, and encompass different styles of atmospheric music inspired by classical and folk. Aquilus' first proper release came out in 2007, in the form of the EP titled Arbor. After many years of writing and composing, Horace finally released Aquilus' debut opus in the end of 2011, which was titled Griseus.