Quick answer — "Aria" by Hirasawa is the opening theme of Berserk: The Golden Age Arc III - The Advent and 2 other anime.
"Aria" by Hirasawa is the opening theme of Berserk: The Golden Age Arc III - The Advent, Berserk: The Golden Age Arc II - The Battle for Doldrey, Berserk: The Golden Age Arc I - The Egg of the King.
"Aria" is performed by Hirasawa.