The 1991 metal album "Necroticism - Descanting the Insalubrious" [0] by British band Carcass was a landmark work which took the use of unusual vocabulary in metal lyrics to a new (and arguably-satirical) level [1]. Here are some examples, lists of the uncommon words included in a few of the songs lyrics:

Track 1, "Inpropagation" : atrabilious, crevet, ebullition, beatrific, exequiet, coronach, humous, fecundate, defalcating, lacrimation

Track 2, "Corporal Jigsaw Quandary" (see [2] for YT video of song) : evulsed, deviscerated, chondrin, decollate, quiescent, detrital, sequacious, commensated, convented, uliginous

Track 4, "Pedigree Butchery" : spumous, crubescent, martilinear, desparental, primparal, sporulate, chimerical, rheological, fluxing, "illegitimeat", contumely, ebullient, despumation, desipient

Back then it was common belief that the band members were medical students, and that the terminology in their lyrics came from their studies, but this ultimately turned out to be a myth.

[0] https://en.wikipedia.org/wiki/Necroticism_%E2%80%93_Descanti...

[1] http://www.darklyrics.com/lyrics/carcass/necroticismdescanti...

[2] https://www.youtube.com/watch?v=F5nGo7yUtlE