XV Episode 6
An attempt at recreating an ancient power once thought to be held by vampires, just as the name suggests, it allows the user to forcibly interfere with others minds via a cursed gaze.
It's difficult to use on someone of disciplined will and requires taking advantage of the target's weaknesses or emotions, and so can't be considered a very effective weapon.
Unless, of course, it can be mastered.
In that case, it becomes a fearsome evil eye.
It can corrode the heart, impacting not only judgment but also self-esteem.
This in turn can increase the target's dependence on others, causing them to exhibit a number of effects similar to suggestion or hypnosis, such as forcing them to carry out a command.
Swallowed by the rage from having witnessed such a tragedy, Tsubasa allowed herself to fall victim to her opponent's gaze, causing a seal to be carved deep into her heart.
Said seal is exploitable not only by the one who placed it, but also third parties.
Using a trigger phrase, it is easy to seize control of her consciousness.
the one using that trigger is the heretic Kazanari Fudou himself.
XV Episode 9
In order to have his commands carried out, he would activate the seal that had been placed on her.
However, at the time that he attempted to use it, the applier of the seal, Millaarc, was gasping on the edge of death, and the strength of the seal began to waver.
And so, the details of what had been whispered to her and what she had done came back to Tsubasa, becoming yet another wound that trampled on her pride as a sentinel.