Sleepwalker - [Sleepwalker II #02] Wolf of Truth and Dream [7:59]
I wish I could link to a video of this but there's pretty much no chance of this being uploaded anywhere because it's way too obscure, and the fact that the only digital release of this material didn't have song titles doesn't help either. Truly eerie black metal/post-rock. For people who like this type of music I also strongly recommend Fell Voices and Ash Borer, both of which are related projects (although Ash Borer has two degrees of separation).
Ihsahn's vocals are amongst the best in progressive metal, but he's had classical training so I guess that shouldn't be much of a surprise. He's honestly almost up there with vocalists like Rob Halford, Bruce Dickinson, King Diamond, etc., in terms of the quality of the cleans, and is arguably more versatile due to his wide range of screaming styles.
His solo stuff isn't as great or historically important as his work in Emperor but it's still tremendously interesting material.