Complex sentence: For the most part it wasn't until the release of the album that it was appreciated de facto, up until that point, "Howling Bells" were still very much unknown, for the vast majority of followers yet to be, at least.
Split into simpler sentences: For the most part it wasn't until the release of the album that it was appreciated de facto. Up until that point "Howling Bells" were still very much unknown, for the vast majority of followers yet to be at least.