From Yu-Gi-Oh!

A Mark of the Shadows (known as a "dark mark" in the Japanese version) is a mark appearing on a Dark Signer's arm.

Each mark corresponds to one of the seven Earthbound Immortals. This mark was given to Roman Goodwin, Devack, Kalin Kessler, Misty Tredwell, Rex Goodwin, Greiger, and Carly Carmine by the evil Gods themselves, when they were close to dying. Roman obtained his Mark of the Shadows from the core of Original Enerdy Reactor itself. It is unknown how Devack gained his powers. 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.

Spider mark

Having the dark mark allows the Dark Signer to use the corresponding Earthbound Immortal as a card.

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.