According to Taylor Gorrell, "D/D/D/D" is short for "Different Dimension Daemon Dragon" (ディファレント・ディメンション・デーモン・ドラゴン Difarento Dimenshon Dēmon Doragon), and as such, all "D/D/D/D" monsters have primarily dragonic forms and are treated as Dragon-Type monsters in addition to their printed Fiend-Type.
All "D/D/D/D" monsters are evolutions of the "D/D/D" monster respective to their secondary Type, sharing their Attribute and being Summoned by the evolution of whatever Summon their original form was Summoned by, usually requiring their original form as a Material. Their actual names are two words, the first word being a synonym for the Underworld or Hell from a different mythology, and the second word being a word related to their element or otherwise.
|D/D/D/D||Underworld (Mythology)||D/D/D||Summon Evolution|
|Flame Dragon King Diyu Blast||Diyu (Chinese)||Flame King Genghis||Contact Fusion|
|Wave Dragon King Duat Flood||Duat (Egyptian)||Wave King Caesar||Xyz Evolution/Rank-Up|
|Shadow Dragon King Hades Shade||Hades (Greek)||Shadow King Pizarro||Unnamed Hybrid Summon evolution|
|Unnamed "D/D/D/D" monster||Hell (Christianity)||Doom King Armageddon||Unnamed Pendulum Summon evolution|
|Peak Dragon King Naraka Quake||Naraka (Buddhism)||Ashoka the Peak Overlord||Unnamed Ritual Summon evolution|
|Gust Dragon King Tartarus Storm||Tartarus (Roman)||Gust King Alexander||Accel Synchro Summon|
|Sun Dragon King Yomi Shine||Yomi (Shinto)||Sun King Cortez||Super Composition Summon|