unchanging for all eternity
First, you need to understand the nature of the zodiac’s curse as well as its origins.
The life of each zodiac spirit takes place in the same room, in the same space, and at the same pace. Nothing can be taken away from us. We remain unchanging for all eternity. A happy future awaits us. A never-ending banquet… unchanging, eternal. Akito, Volume 11
Hundreds of years ago, “God” held a banquet with thirteen animals. Realizing that each of them would die at some point, regardless of his feelings for them and how much fun they had together, he cast a spell to make that friendship eternal. Every time they’d die, they’d be reborn to hold a banquet once more; there’d be no need to be afraid of death or of loneliness, for the promise would ensure that they’d meet again and again. That first promise has since been forgotten.
Akito, the head of the Sohma clan and the equivalent of their God, was promised said eternity since childhood. Her father entrusted her with the bond to the zodiac, and ensured her that she was something special — born to be loved. His death, however, further increased the rift between his wife Ren and Akito. An obsessively jealous Ren decried the authenticity of the bond between the zodiac and Akito, calling it fake and delusional, whereas Akito interpreted it as true love, just as her father had said. Determined to prove Ren wrong, Akito’s personality became twisted and controlling, doing just about anything to cling to the members of the zodiac.
The physical and emotional abuse she inflicts on them are, from her perspective, expressions of love, meant to keep them in check and by her side. An outsider may judge differently, but it is in the very nature of the bond that it shuts everyone out: Subconsciously, bound by the promise and by their blood, the members of the zodiac regard Akito as a God that they fear and revere, unable to resist anything that she does.
By the same token, the idea of an eternal bond may be considered something beautiful on the surface, but to someone who thinks of it as a burden, it becomes a prison: This holds true for pretty much all members of the current zodiac, especially those abused by Akito, as it creates a barrier between them and everyone else (in part due to the transformation effect). That eternally repeating bond is the true curse.