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The Subterror archetype, a series of EARTH attribute Flip effect Monsters, was first released as a TCG-exclusive archetype in The Dark Illusion in 2016. Subterrors consist of Level 4 and below Monsters with effects to supplement their main deck boss Monsters, the Level 5 and above sub-archetype Subterror Behemoths. Loading... , quite possibly the strongest support ever printed for the archetype, was released in Savage Strike three years later in 2019.
Subterror Guru, unlike pure Subterror, is a control deck revolving primarily around Loading... and setting multiple disruptive Traps like Loading... and floodgates like Loading... . As a deck revolving primarily around one main deck Monster, archetypal disruption, and floodgates, another deck may come to mind...
Why Not Eldlich?
Eldlich is a tiered control deck with a similar strategy, while Subterror Guru does not compete in the current metagame in quite the same way. However, there are several merits to the Guru strategy that Eldlich misses out on. This will be covered more extensively in the card-by-card descriptions, but essentially, certain cards are much more accessible to Guru than they are to Eldlich. As such, the options that other decks incorporate to play around floodgates standard to Eldlich are not as effective against Guru, and duelists on the ranked ladder may not understand how to play around Guru as well as Eldlich due to the lower amount of representation.
Subterror Guru (x3)
The namesake of the deck himself. You want to see this guy as much as possible, so you'll need to run three copies to play this strategy effectively. Keep in mind that each of the following effects have a hard once-per-turn on them, so you won't get extra value from having multiple Gurus on the field.
Loading... allows you to add any Subterror card from your Deck to your hand when he's flipped face-up. This will be your primary method of filling your hand with what you still need, particularly Loading... and Loading... .
Guru also has the effect to flip himself and another Monster on the field into face-down Defense Position. In addition to potentially setting a problematic Monster on your opponent's side of the field, this is also a common way you will find yourself resetting Loading... so that he can trigger his effect to search a Subterror card by flipping up again. This effect is strongest as a Quick Effect, available only when you have another Subterror card face-up on the field, since you can chain the effect to your opponent's effects and use it on their turn.
Subterror Fiendess (x3)
Loading... is the only effect negation native to the Subterror archetype. You generally want to make sure you have at least one of these in-hand at all times, and as such tends to be the highest priority target for Loading... 's effect. Encountering her in your opening hand is especially important for when you don't have access to Loading... on your first turn, which makes it important to run three copies. Each of her effects have a hard once-per-turn as well.
Loading... is an incredibly powerful omni-negate hand-trap which requires you to have a face-up Subterror Monster on the field. It activates by sending itself from the hand or field to the GY (though primarily used from the hand), targets the Subterror Monster, negates the activation of an opponent's card effect, then flips your Monster face-down. Because the activation is negated, unlike with an Loading... , you can negate a Continuous Spell/Trap and it will be sent to the GY instead of remaining face-up on the field with its continuous effect applying. Loading... is another method with which you can reset your Loading... .
As powerful as Loading... is as an omni-negate, there are several downsides to keep in mind when using it. First, because it targets a Monster on the field as part of its activation, the effect can whiff if the targeted Monster is removed from the field or Set face-down before the effect resolves, meaning you don't get your negate. Second, unlike hand-traps like Loading... , Fiendess specifically must send itself to the GY; this can result in the card being unable to activate under the effect of Loading... , Loading... , or even our own card Loading... .