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"What is Eternal" uses Beethoven's 9th ("Ode to Joy") and Moonlight Sonata. Their "Mephistopheles" also uses Moonlight Sonata.
This forum (http://rateyourmusic.com/list/emperorsakis/beethovens_moonlight_sonata__op_27_no_2___mvt_1__in_metal
) has a list of a lot of metal songs that use Moonlight Sonata. Most of them include video/music. That might help narrow down your search.
Unfortunately The link's down. But that's it. I don't know why I didn't realize it was just Moonlight Sonata. Thanks.
All that and a toilet.
I think there's a song that sounds very similar to "What is Eternal" by Trans Siberian Orchestra (Not the Ode To Joy Part) I don't really know much about it. I think it may have been another metal song.