additional clarification to the screenshot above.
Triss's unusual skull shape can be attributed to Triss's fatal head injury during the Battle of Sodden Hill. The shock wave and heat wave from a magical explosion (in combination with barotrauma) caused Triss multiple injuries to all internal organs, fractures of the bones of the skeleton and skull.
Such injuries are characterized by irreversible, fatal damage to the sphenoid and temporal bones and, consequently, damage to the most important parts of the brain, burn damage to soft tissues, especially the tissues of the auricle, lips and nose.
And what can be seen on Triss's skull?
Indeed, the area of the sphenoid and temporal bones on Triss's skull is completely lost. In case of critical injuries, the structure and morphology of these bones does not allow reconstructive surgical manipulation, therefore, this damaged area of the skull is completely removed. This led to a change in the shape of the skull, Triss's skull became spheroid. This form is not characteristic of the anthropoids of the species Homo Sapiens.
Probably, the special "incantation" returns to the "normal", anthropoid appearance of Triss's skull, this "incantation" was developed individually for Triss - its action - the orbital area rises slightly, the lower, jaw part drops down and protrudes slightly forward.
The auricle of Triss's head is extremely hypertrophied, and bears clear traces of extensive reconstructive surgery, or it is ears transplanted from another donor.
That is - on Sodden Hill Triss really died during the battle. But unlike Litta Neid, Triss's life was a very important key link in the Destiny string and she was "brought back" to life. But the level of medical and magical manipulation used for this incredible event provides a clue to the essence of the "invisible" part of the entire story in the Witcher universe.