Declan Akaba with "D/D Berfomet", "D/D Necro Slime" and "D/D Ghost".
The regular "D/D" monsters seem to be based on different figures and monsters from Western mythology, while the "D/D Savant" monsters are references to great natural scientists from Western history.
In contrast, the "D/D/D" sub-archetype features a distinct design style from the rest of the archetype, with many of its members modeled after famous conquerors from world history.
Taken together with its subordinate and sub-archetypes, the "D/D" archetype features a mild "business" sub-theme (probably to fit the profession of their owner in the anime), with many support cards referencing elements of modern big business. For example, the "Dark Contract" cards resemble business contracts, and the names of "D/D/D Human Resources" and "Contract Laundering" reference like-named company practices.
Additionally, some of the archetype's boss monsters such as "D/D/D Doom King Armageddon" feature a pun in its Japanese name– "Great Des (Death) King" is scripted as "Shiiō", a homophone for "CEO". A similar pun can be found in the Japanese name of "D/D/D Oblivion King Abyss Ragnarok"/"D/D/D Wave Oblivion King Caesar Ragnarok", "Oblivion King", which is scripted as "Kaichiō", a homophone for "Chairman".
So far in the ARC-V manga the "D/D" monsters appear to be demons with a mechanical, scientific theme (distinct in design to the "Savant" monsters) and have equipment like scales or microscopes instead of faces.
This archetype's primary play-style is to use the beneficial effects of the "D/D Savant" monsters and "Dark Contract" Continuous Spells and Traps to gain card advantage: while trying to avoid the LP damage with the Field effects of "D/D/D Oracle King d'Arc", "D/D/D Rebel King Leonidas" or remove the "Dark Contract" cards from the field (after completing the card's combo) with the Self Reviving effect of "D/D Lamia".
When Normal Summoned "D/D Savant Kepler" can search for any "Dark Contract" card, "D/D Swirl Slime" is has a useful effect, from the hand, for loading the Graveyard for revival targets for later and can Banish itself from the Graveyard, on the same turn, Special Summon any "D/D" Monster from the Hand.
The Monster Cards "D/D Savant Copernicus" and "D/D Necro Slime" can be searched by the effect of "Dark Contract with the Gate" and both have effects that allow themselves and other D/D to be used as Fusion Material to Summon the Level 6 usually first "D/D/D Flame King Genghis", Level 7 Panic Button "D/D/D Oracle King d'Arc" and Level 8 beater/rank 8 fodder "D/D/D Dragonbane King Beowulf".
"Genghis" can be used as Synchro Material for "D/D/D Gust King Alexander" with "D/D Lamia"; these three Monsters can now revive each and be used for two of the most powerful Synchro Monsters called "Crystal Wing Synchro Dragon", "D/D/D Cursed King Siegfried" and sometimes "PSY-Framelord Omega".
Both "Abyss Ragnarok" and "Kepler" are recoverable with the Pendulum Effect of "D/D Savant Thomas", and the Monster Effect of "Thomas" is the simplest way to search for a Level 8 "D/D/D" Monster and make "D/D/D Duo-Dawn King Kali Yuga".
"Caesar" and "d'Arc" were also one of the earliest TCG (before "Alexander" and "Genghis" were released) combinations used to abuse the recovery effect of "d'Arc" and the effect of "Caesar" to add a "Dark Contract" card from the Main Deck to the hand.
Additionally, the Deck has access to many cards that can recycle resources from the Graveyard, face-up from the Extra Deck, and the Pendulum Zones, such as "D/D Lilith", "D/D/D Oblivion King Abyss Ragnarok" and "D/D/D Human Resources", which requires the player to carefully utilize resources from many different regions of the field.
The "D/D/D" archetype is a sub-archetype to the "D/D", and consist of more powerful monsters, including Fusion, Synchro, Xyz, and Pendulum Monsters. The "D/D" monsters are used to facilitate the Summons of the "D/D/D" monsters. Examples are "D/D Nighthowl" and "D/D Berfomet", which make Synchro and Xyz Summoning much easier by Summoning the required Material from the Graveyard and manipulating the Materials' Levels, respectively.
Similar to the field effect of "Thomas": "D/D Berfomet" can change the Level of a "D/D" monster to anywhere from 1-8, but restricts Special Summoning to Monsters from this archetype. The only Xyz Monsters for this effect are "Duo-Dawn", "D/D/D Wave King Caesar" or D/D/D Marksman King Tell.
With the release of Structure Deck: Pendulum Domination, "D/D Savant Copernicus" and "D/D Savant Newton" provide more consistent Pendulum Scales than "D/D Savant Kepler" and "D/D Savant Galilei". "Copernicus" has the bonus of dumping "D/D" cards from the Deck to start combos, whereas Newton can discard itself to add a "Dark Contract" card or a different "D/D" monster from the Graveyard. "D/D Orthros" is a reusable Tuner monster, by virtue of also being a Pendulum Monster, to make Synchro Summons easier to execute. "Dark Contract with the Yamimakai" can set the Pendulum Scales from the Graveyard or Extra Deck to start Pendulum Summoning.
In the manga, "D/D" has a vastly different playstyle. Unlike its anime counterpart, it doesn't focus on Fusion/Synchro/Xyz sSmmoning or negating effect damage from the "Dark Contract" cards. Instead, it focuses on reducing the DEF of the opponent's monsters to 0 (reflecting Reiji's obsession with the number zero in the manga) and dealing piercing damage. However, it does make use of Pendulum Summoning.
Because this Deck focuses on swarming to push the game, cards that can disable or counter swarming or searching, like "Fossil Dyna Pachycephalo" or "Thunder King Rai-Oh", can greatly hinder this Deck. Additionally, the archetype's synergy with the "Dark Contract" archetype provides a weakness to Spell/Trap removal, as well as a dependence on cards that mitigate the damage from the "Dark Contract" cards.
The following Decklists may or may not contain copies of cards that exceed the maximum amount allowed by the current Forbidden & Limited List. These should not be changed, as these Decklists are copied verbatim at the time they were published.