Classical music weaponized into battle themes, and it goes way harder than a 1997 anime soundtrack has any right to. The Violinist of Hameln takes Beethoven, Tchaikovsky, Mozart and Sarasate and reframes them as literal magic attacks, which is the most anime concept imaginable.
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The eyecatch arrangements of Beethoven's Violin Concerto and Sarasate's Zigeunerweisen hit with this dramatic punch that bridges the original compositions into fantasy action. "Main Theme of the Big Demon King Blood and Flesh" is exactly as intense as that title promises.
Then the Tchaikovsky Swan Lake arrangements go from Dance of the Swan into Raiel's solo and it's genuinely moving. Classical nerds playing dark fantasy. If you love shows where the power system is literally music theory, this is the soundtrack to start with.