A Mark of the Shadows (known as a "Dark Mark" in the Japanese version) is a mark appearing on a Dark Signer's arm, though with some exceptions, as shown with Rex Goodwin's Condor Mark that appeared on his back.
Each mark corresponds to one of the seven Earthbound Immortals. Having the dark mark allows the Dark Signer to use the corresponding Earthbound Immortal as a card.
|Mark of the Shadows||Dark Signer||Earthbound Immortal|
|Spider|| ||Roman Goodwin||Uru|
|Hummingbird|| ||Carly Carmine||Aslla piscu|
|Giant|| ||Kalin Kessler||Ccapac Apu|
|Killer Whale||Greiger||Chacu Challhua|
|Condor||Rex Goodwin||Wiraqocha Rasca|
This mark was given to Roman Goodwin, Devack, Kalin Kessler, Misty Tredwell, Carly Carmine, Greiger and Rex Goodwin by the Immortals themselves, when they were close to dying. Roman obtained his Mark of the Shadows from the core of Original Ener-D Reactor itself. It is unknown how Devack gained his powers for it was never mentioned. All of the the dark marks have appeared on the Dark Signers right arm, except the condor which is instead on Rex Goodwin's back.
The Signer equivalent of the Mark of the Shadows is the Mark of the Dragon. Both identify a person belonging to either group and react to one another whenever a Dark Signer and a Signer are near one another. Also, both marks are active while a Dark Signer and a Signer are facing against each other within a Shadow Duel. The Mark of the Shadows will glow even brighter, if its owner is getting taken over by their Earthbound Immortal (this is evidenced during the Duels with Misty, Carly, and Greiger).