They are completely invisible and not target-able to me. They attack me so I fun my butt off. I lured a guard near one, the guard killed it and that's how I figured out what it was from its corpse which is visible.

Are they supposed to invisible? Is there a spell I'm to cast to reveal them?

A very special brew known as ethereal vision must be consumed to see the spectres. One may seek information about its recipe by finding their way into the heart of the hedgemaze.