Burzum's Daudi Baldrs would be a good example of very
repetitive piano music:Hermodr A HelferdIlla TidandiMoti Ragnarokum
The technique of using repetition to enchant a listener is fundamental to the approach of many foundational black metal and doom metal bands. It's no surprise that it can work with other instruments and other genres.
But why does repetition have this effect (someone asked)? I never really figured out why, but I know it can work on me. "Illa Tidandi" and "Moti Ragnarokum" are two of my favorites by Burzum, despite their simplicity, or maybe because of it:
When listening to music that uses an abnormally great amount of repetition, I follow along normally for the first 4 or 6 reps, and then it gets to a point where my ear's intuition thinks the section will be over, even after having heard that piece of music several times before. Then it continues and that's when the effect kicks in. It has something to do with this feeling of permanence or consistency.
Dunkelheit and Transilvanian Hunger are some of the first songs that made me "get" the use of repetition in this way.
I noticed a similar effect when hearing a live performance of Inno A Satana from one of Emperor's DVD's. At the climax of the song, there are four lines of lyric delivered with a scowl/scream and under it you can hear a prominent snare sound. Passing from the screamed lines to the clean-sung ones the underlying riff and snare continue blasting along, creating that same feeling of "permanence". That's the best I can do to describe it.
Performance of Inno A Satana I was referring to: from Emperial Live Ceremony