Does anyone know what instrument this is?

Sounds an ambient voice on a synthesizer. Unless I’m getting the time period of this movie completely wrong or you’re talking about the swishing of the sword sound.

Edit: Didn’t check the details. Most definitely a synthesizer of sorts.

I don’t mean the wind like swishing sound, but the more dominate unnatural sound that echoes slightly after each swish of the sword.

I think you’re right. Thanks!

I agree with @dverhey, it sounds like a synthesizer.

Where on the video is the specific sound of the instrument you are asking about? The video is 10 minutes long.

@lillycoyote It’s at 5:20 to 5:38 (as the OP posted in the details).

@Seaofclouds Oops! Thanks. I really need to start reading the details, it gets late, I get tired… :)

It’s not a real musical instrument, but it’s a cool audio effect nonetheless. Each tone begins as a flute or possibly pan pipes. Hard to tell if it’s an acoustic recording or some kind of synth flute patch. Each tone then evolves over the next few seconds in a complex way to become a decidedly unnatural sound.

I hear two separate processes: (1) Overtones of the fundamental are gradually boosted to dominate the sound—like some guitar pedal effects. Or you could apply gradually-changing EQ filters to cut or boost various pitches. (2) The sound is chopped in time—like an effect I know as “flanging”. Probably there is computer-based sampling of brief audio snippets. Rate, period, and duty cycle of each snippet could be manipulated to make that staccato echo effect. It’s more sophisticated than just reverb, though I’ve heard feedback loops that can evolve in weird ways, especially if carefully controlled.

@gasman I heard more of a pad synth instead of a flute patch, myself, possibly run through a flanger of sorts. But perhaps that’s just me.