So a while back I was searching up Kamelot'd discography online, and apparently the chorus of the song Forever derived its melody from Solvieg's song by Edvard Grieg, as stated by Roy Khan.




Which gets me wondering, are there any other metal songs that where a melody is directly taken from a classical piece that isn't an explicit cover of said piece?

The whole song is Solveig's sang. It's only the lyrics that are changed.

Classical Music gets worked into songs all the time.

One of my favorite examples is Angra - Evil Warning that has Vivaldi's Four Seasons and Lasting Child has a Mendelssohn piano piece.

Savatage worked Grieg into Hall of the Mountain King.

The first Gamma Ray has a part of one of Beethoven's symphonies worked into one of their songs.

The first Armored Saint album opens with part of Mussorgsky's Pictures at Exhibition.

There are many more but I would need to go back and listen to the albums to remember the actual song/s.

Mekong Delta does the whole Pictures at an Exhibition.
https://www.youtube.com/watch?v=Wzbrsx8C6bY

Symphony X used part of Mozart's Requiem in The Death of Balance. They do have little pieces inserted into their music here and there.

I am mad at myself for not having a better memory right now!

Galneryus' Angel of Salvation is in part an arrangement of Tchaikovsky's Violin Concerto in D major, Op. 35

Primordial - Gallows Hymn (Beethoven's Moonlight Sonata)
(edit: sorry, I only read the title and didn't listen to the example in the OP. Looks like my example doesn't fit into the thread)

Tyr also uses the main theme from Solveig's Song in the track "Valkyrjan" from Land.

i can't seem to remember what piece is used and what track it's in, but necrophagist did this on at least one occasion - as a brief part of a solo or something it was

Cult of Fire have Vltava, a black metal version of Smetana's Moldau

Cult of Fire:
https://www.youtube.com/watch?v=jTlapVqx0hs&ab_channel=CultOfFireofficial
Original:
https://www.youtube.com/watch?v=3G4NKzmfC-Q&ab_channel=RichardBrittain

Yep On Epitaph album

The Stillborn One Beethoven
Only Ash Remains Profokiev

"The Eye Collector" by Krypteria is basically half original song, half "Moonlight Sonata." Including the 2nd movement that people often forget exists.

Children of Bodom did this a lot on their earlier albums.

Red Light in My Eyes, Pt 1 (Two Part Invention in A Minor - Bach)
Red Light in My Eyes, Pt 2 (Symphony No. 25 - Mozart)
Red Light in My Eyes, Pt 2 (Confutatis from Requiem Mass - Mozart)
Hatebreeder (Magic Flute (Queen of the Night Aria) - Mozart)
Black Widow (Piano Concerto No. 20 - Mozart)

A lot of the guitar solos on the first 3 Children of Bodom albums have parts of classical songs in them. For example, the solo of the song Children of Bodom.

Surprised no one's brought up the neoclassical metal groups like Cacophony and especially Yngwie Malmsteen. Those guys bring in a lot of classical influences in their riffing.

when Botch was more hardcore than what they'd become, they did a cool version of "O fortuna"

Is this the stuff that you are looking for?

A handful of Accept songs (esp instrumentals) are of classical music, I think Wolf has done at least one solo album that is based on classical music.

Marduk's Glorification of the Black God is based on the Night on a Bald Mountain by Mussorgsky

Cirith Ungoal-Toccata. Based on JS Bach's Toccata and Fugue

Savatage-Mozart and Madness and In Memory.

Someone above mentioned Mekong Delta's Pictures at an Exhibition, but that's only a small representation of how many of the band's songs are covers, reinterpretations or influences from classical music, here's a short list of some of my favourites:

Toccata


Night on a Bare Mountain


Landscape 4 - Pleasant Ground

Golden Resurrection - One Voice for the Kingdom the song Spirit War has Mozart's Rondo Alla Turca in the guitar solo.

Samael’s “Baphomet’s Throne”. Its contains elements based on a piece my Modesto Mussorgsky.

A killer song from a perfect album.

Eclipse - Wired; the song Twilight has Beethoven's Ode to Joy from the 9th Symphony. Kicks in at about 3:15 and is well done.

Megadeth's Last Rites is an interpretation of a Bach classic.

I'm not sure if it's really classical in the true sense, but Metallica's Frayed Ends of Sanity opens with an interpretation of The March of the Winkies from the Judy Garland classic Wizard of Oz.

Hollenthon have used Mussorgsky’s “The Hutt on Fowls’ legs” as the intro for “On the Wings of a Dove” and continued the theme in the riffs. Great stuff. It took me 15 years to discover the intro was a classical piece and not written by the band.

Another great example of classical music matching the feelof a specific subgenre is Dvořák's 4th movement in his New World Symphony, which Rhapsody have used on their Rain of a Thousand Flames ep.

(start at 39:37)