One Piece features an abundance of exciting mysteries that add to the lore of the series and inspire a plethora of theories. The latest addition to this long list is the Mark of the Abyss, which has played a significant role in the Elbaph Arc.
The Mark of the Abyss is a power that is presumed to be granted by Imu to the God's Knights and the Gorosei, appearing as a distinguishable symbol on their body. It allows its users to freely use the Abyss, an unknown concept, to instantly transport across great distances using Magic Circles.
The abilities of the Mark of the Abyss were first hinted at during the Egghead Arc, during which Saint Saturn summoned the rest of the Elders using the Abyss. These Magic Circles created an outburst of black lightning, much like Haki. However, the Elbaph Arc has explained the Abyss in much closer detail.

As mentioned, holders of the Mark of the Abyss can instantly transport through Magic Circles. However, individuals can also be summoned without their consent and forcefully pulled to locations, provided they have the mark. So far, only Saint Saturn and Gunko have displayed the ability to create the Magic Circles associated with the Abyss.
Additionally, those who do not have the mark cannot travel through the Abyss. Since the God's Knights couldn't bestow the marks upon the Giant children they were kidnapping, it reaffirms that the power is likely granted by Imu.
The Abyss also has a demonic connection and may even represent hell itself, as Imu's Domi Reversi spell can turn enemies into demons loyal to their command by flipping them through the Abyss.
Theories about the Mark of the Abyss in One Piece
Apart from the confirmed powers of the Mark of the Abyss in One Piece, there are countless theories surrounding its true abilities, many of which have been strongly hinted at. The first and most likely possibility is that those with the Mark of the Abyss can be controlled by Imu, potentially even having their bodies taken over.
During the God's Knights’ attack on Elbaph, Imu notably descended by taking over Gunko’s body. Their appearance caused a fearsome emanation of Haki similar to what occurs with the Magic Circles. This belief is granted more weight through the recent revelation that Shanks himself seemed to have the Mark of the Abyss in the past.
However, he coincidentally lost the same arm that had the mark when saving Luffy from the Sea King, which may have freed him from Imu's control. Therefore, Shanks’ sacrifice to protect the protagonist could have been both heroic and self-serving.
If those with the Mark of the Abyss are similar to Imu's demons, except for the physical changes and overt manipulation, it could also explain the immortality of the Gorosei and the God's Knights, as well as how Saturn was killed by Imu earlier in One Piece.
While Oda has yet to confirm the true purpose that the Abyss and the bearers of the mark play in One Piece, it is clear that there is more to it than meets the eye. With subtle clues suggesting that the Abyss may be responsible for major events in the series’ world, fans are eagerly looking forward to uncovering more about it.