Paramaecium - The Unnatural Conception in Two Parts: The Birth and the Massacre of the Innocents
This is one of my favorite bands, and this song is the opener of my favorite album by them. It may be a bit of a stretch to call this classical, but it uses some classical instrumentation, so I'm counting it. This is one of the quintessential doom metal albums from the 90's, and IMO belongs in the same conversation alongside My Dying Bride and Cathedral. It's certainly got more of a death vibe vocally than those other two bands, but they definitely nailed the drudge/sludge/funeral overtones that later bands would emulate.
It's a long song, but do yourself a favor and give it a chance.