The Thing That Should Not Be is the third song of Metallica's third album, Master of Puppets, released in 1986.
Cosmic Horror Inspiration
The lyrics of "The Thing That Should Not Be" seem to fit well with the H.P. Lovecraft Cthulhu mythos, including lines such as "Great old one" which may be a reference to the Great Old Ones often appearing in Lovecraft's works, "Hybrid children watch the sea, Pray for father, roaming free" which may refer to the people of Innsmouth (a key location in the Cthulhu Mythos) and "Out from ruins once possessed, Fallen city, living death" a possible reference to Cthulhu himself as he resides in R'lyeh (another key location in the Cthulhu Mythos).
The lines "Not dead which eternal lie/Stranger eons death may die" is a phrase referencing the couplet "That is not dead which can eternal lie. And with strange aeons even death may die." that appears in the fictional Necronomicon that Lovecraft references numerous times.
The lines "Messenger of fear" and "Crawling Chaos" refer to Nyarlathotep, the messenger of the evil Outer Ones. The line "Cult has summoned" may refer to the Cult of Cthulhu and their quest to awaken their god.
Tuning
The studio recording is their first in D standard tuning; however, it is their only D-tuned song to be played live in D-flat tuning.