Open an edit with this choir under the approach, then cut to the Inception Horn Sound Effect at the moment the boss is fully revealed to heighten the climax. Follow it with a short impact hit for the first attack, keeping the choir as the opening layer rather than the payoff itself.
About this sound
This is a slow, wordless male choir chant with deep, sustained voices rising and falling over a hollow, cathedral-like echo. It feels vast, solemn and threatening, as if something enormous and timeless is approaching, and it turns a plain entrance into a moment of dread. The passage comes from Dark Souls, whose boss encounters made this kind of choral music a signature of dark fantasy scale.