A tattooed third eye on Carl's sternum that he can open to see through, holding vast and dangerous power.
After the procedure, Carl discovers he can see from the eye on his chest. It is still a tattoo, but he can feel it when it is open, and keeping it shut takes a conscious effort. The doubled vision makes him dizzy and nauseated — part of why his eyesight feels wrong the moment he wakes in the ruined ballroom.
He tries opening it deliberately, then clenches it shut a moment later. There is power there, so much power, and he judges that he has to keep it closed. It is the visible interface of Shi Maria's presence inside him, the egg Hapanzi sealed, and Carl reckons it alongside Carl's Doomsday Scenario in his growing collection of weapons of mass destruction.
A tattooed eye on the sternum, indistinguishable from ink when closed but tangibly open when used, granting a second field of vision.
Carl's new and barely controlled sensory link to the demigod trapped inside him.