Clockwise from center: "Defender", "Warrior" and "Archer" are attacking "Stalagmo" and "Voltelluric" in the artwork of "Subterror Cave Clash"
"Subterror" is an archetype of EARTH monsters that first appeared in The Dark Illusion. It is one of two TCG-exclusive archetypes that premiered in that set, the other being the "SPYRAL" archetype. The archetype includes a sub-archetype, "Subterror Behemoth".
While all members of the "Subterror" archetype are EARTH, each one has a Level and Type unique to it. The archetype's higher Leveled monsters belong to the "Subterror Behemoth" archetype, which is comprised of colossal beasts whose names are a reference to subterranean anomalies or types of rock. The lower Leveled monsters comprise the "Subterror Nemesis" sub-series, who are human hunters who confront the "Subterror Behemoth" monsters.
The effects of the two "Subterror Nemesis" monsters are very necessary in order to Special Summon the "Subterror Behemoth" monsters from the Deck face-up or face-down. Since "Subterror Nemesis Warrior" has the ability to be continually re-used, it is common to see the cards "Speedroid Terrortop" and "Speedroid Taketomborg" being used to Xyz Summon "M-X-Saber Invoker" simply to search it out. "Tour Guide From the Underworld" with any Level 3 Fiend-Type monster or "Predaplant Ophrys Scorpio" with "Predaplant Darlingtonia Cobra" can have the same result, with the ability to search "Instant Fusion" if using the "Predaplant" engine.
Flipping the "Subterror Behemoth" monsters face-up and face-down can be controlled through card effects such as "Prediction Princess Tarotrei", "Book of Taiyou", "Subterror Final Battle" and "The Hidden City".