Subterror Behemoth

Every Subterror Behemoth shares the same two-part text: when a face-up monster you control is flipped face-down and you control no other face-up monsters, the Behemoth Special Summons itself from the hand in face-down Defence Position, and once per turn it can flip itself back down. The engine exploits this loop continuously. Subterror Behemoth Fiendess ties it together — during the Main Phase it sends a Flip monster from the Deck to the Graveyard and Special Summons a monster from the hand face-down into its linked zone, then recovers a Flip monster to the hand whenever that pointed-to monster is flipped face-up. In practice this means Fiendess can load the Graveyard, plant a Behemoth face-down, trigger its FLIP effect, and search a replacement all within a single turn sequence.

The FLIP effects themselves cover a wide range of disruption and resource generation. Subterror Behemoth Umastryx banishes an opponent's monster outright, Ultramafus forces every other face-up monster on the field face-down, Voltelluric steals a Set monster until the End Phase, and Speleogeist zeroes out a monster's ATK after forcing it to Attack Position. Stalagmo replaces itself with two draws after discarding a Subterror, and Stygokraken destroys Set cards equal to the number of Behemoths present. Subterror Behemoth Burrowing supports the set-up by flipping a Behemoth face-down at will from the Graveyard and shielding face-down monsters from targeting and destruction, while also searching a Subterror if it is destroyed — giving the deck meaningful insulation against spell and trap removal.