Dark Synchro Monsters
From Yu-Gi-Oh Card Maker Wiki
Dark Synchro Monsters are an anime-only monster category that debuted in Yu-Gi-Oh! 5D's Yu-Gi-Oh! 5D's - Episode 027. Dark Synchro Monsters are Dark Synchro Summoned by sending to the Graveyard 1 Dark Tuner monster and 1 non-Tuner monster. Besides the "Dark Tuner Monster", you can use any monster for the Dark Synchro Summon. The non-Tuner Monster's Level minus the Dark Tuner Monster's Level must be equal to the Dark Synchro Monster's Level (note that the Dark Synchro Monsters have a negative level, something that isn't possible with any other monster in the game). Also, their levels start at the opposite side of the card (regular cards start at the right side, they start at the left side).
Dark Synchro Monsters and Dark Tuner monsters are not simply DARK Synchro Monsters, but are a completely different monster category, although they are similar to Synchro Monsters. The color scheme of that is opposite of a Synchro Monster, with a black card frame.
Dark Synchro Monster's effects usually involve either reviving monsters in the graveyard or preventing their own destruction.
Dark Synchro monsters function as Synchro Monsters with negative Levels for any game-related purposes when Yusei Fudo "Synchro Destructor" dealt damage twice instead of just once because he destroyed Kalin "One-Hundred Eye Dragon" with his Stardust Dragon.
There are no Dark Synchro Monsters in the OCG/TCG. However, 3 cards from this category are to be released as regular Synchro Monsters instead with positive Levels. They are "One-Hundred Eye Dragon", "Underground Arachne" and "Demonic Monkey King Zeman".
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