An endless number of concept albums have been released over the past few decades - the genres of rock and metal have been providing some of the best, worst and outright strangest concept albums out there; in regards to their subject matter. This list focuses primarily on the latter, so it should come as no surprise that there's some pretty crazy stuff to be found below.
Nocturnus - "The Key"
Some old school death metal, and an excellent record in its own right, "The Key" has some absurd elements to be found within. Released in 1990, "The Key" is Nocturnus' debut album. The concept is rather strange, however - it tells the story of a cyborg that travels in time and destroys Christianity through killing one baby Jesus, and then elaborates on the impacts of this. A great record, all in all.
The Protomen's entire discography
The Protomen are a hard rock band, with rock opera influences, that have released two concept albums based on the videogame character of Mega Man. They developed their own version of the lore built from the games and, coupled with an underlying dystopian theme, expanded upon previously told stories in their self-titled debut and sophomore effort "Act II: The Father of Death" respectively.
Alestorm - "Captain Morgan's Revenge"
Alestorm's are primarily a power metal band - with some intricacies that have led to their fans dubbing their genre "pirate metal." Their debut record is a concept album based on the burdens and benefits of being a pirate. Drinking, sailing, buried treasure, this album has it all.
On a slightly unrelated note, it's also worth checking out Alestorm's cover of the song "You Are a Pirate."
Devin Townsend - "Ziltoid the Omniscient"
This concept album is a compelling tale of woe - an alien named Ziltoid travels to Earth in search of the universe's ultimate cup of coffee. Upon arrival he is given what he asks for, but when said coffee isn't to his taste, Ziltoid promptly invades Earth.
A sequel to this album is expected to be released later this year.
The Mars Volta - "Frances the Mute"
The Mars Volta have always been considered absurd but the origins for this album are a tad tragic. The album tells the story of a character named Cygnus and is based on a series of diary entries found by the band's guitar tech. The album was released in 2005 and its closing track, "Cassandra Gemini," which reaches a total of 32 minutes in length, is considered one of the band's strongest ever tracks.
Blind Guardian - "Nightfall on Middle Earth"
The title of this album is telling - the concept is based on J.R.R. Tolkien's "Lord of the Rings" novella, and serves as an excellent power metal accompaniment to the series. Released in 1998, this album is considered one of Blind Guardian's strongest releases.
Lou Reed and Metallica - "Lulu"
This one was presumably expected on the list. The 2011 release was considered one of Metallica's biggest departures from their signature sound. The release itself was based on two German plays written by Frank Wedekind - a tale of a woman who is gradually dragged into the world of prostitution. The album is littered with spoken word sections read by Reed, that help to set the scene. As a whole, the album is just absurd.
Evergrey - "In Search of Truth"
Evergrey's third album, considered by some fans to be their magnum opus, surrounds the topic of alien abductions. This progressive power metal release has its fair share of excellent moments but the lyrical content and the description of the concept might catch some listeners slightly off guard.
Self - "Gizmodgery"
This album's concept is not quite as strange as the others listed - the record's lyrical content revolves around subjects such as childhood. What makes this record different, however, is that it is recorded almost entirely with toy instruments. Perhaps unsurprisingly, this record makes for an intriguing listen overall.
Austrian Death Machine's entire discography
Tim Lambesis' somewhat solo project (featuring Ahhhnold as the impersonator of Schwarzenegger) is one devoted to the filmography of one Arnold Schwarzenegger. Death metal madness with many references to be found from the song titles to the lyrics themselves.
Thank you for taking the time to browse this list. I hope you found said list enjoyable - perhaps you found a few gems in here - and if you have any more suggestions, feel free to name them in the comments!