If you go onto the Wikipedia page for "heavy metal music, you'll find that one of the very first sections of the page discusses the links between metal and classical music and how many guitar players in the hard rock/heavy metal scene, such as Ritchie Blackmore, took influence from classical music when learning how to compose. You'll also find a quote from Kurt Bachmann of American thrash-metal band Believer, who once said that "if done correctly, metal and classical fit quite well together."
It was only a matter of time then, that one day a band would fuse the two together to create a blend. Sure, neoclassical metal became a thing, but that was more to do with the highly technical style of guitar playing present in that music. In terms of fusing the instrumentations of each genre together, Celtic Frost first used symphonic elements on their 1987 album Into The Pandemonium, but the true fusion came in the 1990s, first on Believer's song 'Dies Arae', and then later with bands such as Therion and Within Temptation.
But there can be no doubt that one band stands tall above the rest in symphonic metal: Nightwish. The Finnish band, who released their first record in 1997, have gone from strength to strength as the years have gone by, and despite a number of line-up changes (mostly at the front), their popularity has only continued to grow. They're not perfect; we reviewed their latest album and found it to be surprisingly lacklustre. But no band is, and that doesn't stop them from being the kings of this niche subgenre.
There were a number of albums on the table for this review. Imaginaerum was brilliant, as was Endless Forms Most Beautiful (particularly the gargantuan and jaw-dropping epic 'The Greatest Show on Earth'), but truthfully, only one album stood out: 2004's Once. The last with their original singer, the operatic Tarja Turunen, it stands as the last album of their original style, before a change in singer to Annette Olzon took them down a slightly different path.
Make no mistake: Once is special. With songs such as 'The Siren', 'Creek Mary's Blood' and 'Kuolema Tekee Taiteilijan', half of it sounds like it comes straight out of a fairy-tale, or out of an old Disney movie like Peter Pan. These are the songs that focus slightly more on the symphonies, the melodies, rather than the metal side of things. Those who do prefer their guitars shouldn't fret, however, as they'll be more than satisfied by the powerhouses of 'Dark Chest of Wonders' and 'Romanticide'.