If it was up to Spotify alone, they'd have all the music in the world, and they're working hard to make this happen for you. Sometimes however, agreements can't be reached with the artist or label, or a change may happen in music ownership.
Thousands of albums and artists are added every day. If you can't find (r)evolution of Hammerfall, it may reappear soon. In the meantime, I'd suggest contacting the artist or their label to ask why the album has disappeared from Spotify and if they plan to reupload it.
Hope this has helped :)