This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Black Sabbath
What is this that stands before me?
Their self-titled song. A pure and prime example of "Diabolus in musica." The song was inspired by a nightmare of "Geezer" Butler's. After reading a book about the occult, he fell asleep and woke up a bit later to see a silent creature with a black face staring back at him. And black means pure black. No eyes, mouth, anything — onlyblack.
When it first came out, this song was so scary that the first time they played it, most of the audience ran out of the venue screaming. (The ones that stayed yelled for it to be played again.)
Another common interpretation of the song is that it's about a dabbler in the black arts who inadvertently summons Satan himself. The unlucky summoner is then dragged kicking and screaming down to Hell by the Prince of Darkness.
Listen to the demo version of this song, Ozzy sounds more like a victim of the demonic summoning (even though he was the one caused it, well, technically Geezer... But still) and the extra verse can frighten people.
"Hand of Doom" from Paranoid has frighteningly explicit lyrics about drug addiction, especially in the last part of the song, coupled with occasional, brief bursts of guitar feedback that sounds like a drill entering your skull.
"War Pigs", also from Paranoid. The first and third part of the apocalyptic anti-war lyrics are accompanied with eerie silence save for the regular guitar hits and licks.
The alternate lyrics for the song's first incarnation "Walpurgis" are just as horrific, detailing the disturbing going-ons as Satan's followers sow chaos, death and ruin across the land on their "holy" night.
"Electric Funeral", again from the above mentioned album, is pretty damn scary for a song about nuclear war, with its nightmarish description of the radioactive fallout and the chugging pace of the riff, which feels like some sort of lumbering monster or something.
"Electric Funeral" also contains an unusually terrifying example of the band showing their work. In the third verse, the reference to "terrifying rain" calls to mind the rain storms that followed the atomic bombings of Hiroshima and Nagasaki. Thick, black rain, filled with the ashes of burned buildings and people sucked into the mushroom cloud fell in the aftermath of the blast. Many victims drank the water to, as the song states, "ease the burning pain", not knowing that it was highly radioactive.
The cover of Black Sabbath, pictured above. A figure in black standing in front of a dilapidated house in a wooded area, an image that hits on the same scary imagery The Blair Witch Project covered thirty years later. And who is that? Is it Death? Satan? A witch? Or just a creepy hermit? And what is it staring at?
The haunting whispering and distorted guitar at the end of "Children of the Grave"
"Headless Cross", from the eponymous album, is based on an actual event that happened in a small village in England during the time of pestilence. A group of people went up to a headless cross on a hill to pray to get better, and predictably nobody survives. Martin's vocal delivery and the depressing timbre of the song certainly creates a tragic atmosphere.
From the same album, "The Gates of Hell" really earns its title.
"When Death Calls", from its soft, eerie build-up and haunting themes about fearing death and going to Hell afterwords, up to its louder parts about warning not to stare in "those sunken eyes" and Satan stealing your soul forever. It's goose bump-inducing stuff.
Filler tracks such as "FX" from Vol. 4 and "The Dark" from Born Again. Just give them both a listen in a dark room.
Speaking of "The Dark", it fades into the intro of the next song, "Zero the Hero" where the opening riff sounds like a monster coming alive in the most nightmarish way possible.
"Born Again" from the eponymous album. A slow, haunting, doom metal sound with distorted guitars due to the muddled production give it an atmosphere as though an Apocalyptic Log is recording the coming of the Antichrist.
"Eternal Idol" from The Eternal Idol gives off a similar vibe.
Say what you will about Forbidden, but the opening 50 seconds of "The Illusion of Power" would be a great tone-setter for any Sabbath album.
Much of the final short, "The Drop of Water", but the ghoulish Uncanny Valley face of the dead medium stands out.