Sound Horizon

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Sound Horizon, abbreviated by fans as Sanhora (サンホラ), is a Japanese musical group with composer Revo as the leader.
They call themselves the 'fantasy band (幻想楽団)'.
Their music is generally in omnibus style; an album contains multiple stories with different themes that create one long, big story. Soprano or mezzo-soprano vocals are mainly utilized. Usage of narration is not restrained. Strong anime influences and applications are apparent in their music; they have started inviting seiyū for narration after their major debut. Their lyrics are very often mysterious and metaphorical, even more so than those of other J-rock musicians, with themes varying from fantastic themes like ancient curse, legend, dream, war, paradise to forbidden lust, longing, obsession, crime, relationship, etc.

(Source: MangaUpdates)