After years of nurturing the deification of Ishgard’s dounding father, Archbishop Thordan VII succeeded in infusing his own form with the soul of King Throdan. The ritual used to imbue the archbishop’s mortal vessel with the divine essence of his ancestor followed the same principle as that empoyed by Ysayle and her summoning of Saint Shiva - a rite previously perfected with the Heavens’ Ward and their primal incarnations as the legendary knights twelve.
Drawing upon the incalculable might of one of Nidhogg’s eyes and bolstered by a thousand years of unquestionable faith, the manifestation of King Thordan far axceeded the power of any existing primal. But driven by his monumental ambitions, the archbishop sought to absorb the infinite energies of the Warring Triad and thus ascend to true divinity. As an eternal god-king, Thordan meant to convert all to his cause, be they man or dragon, and “bless” the world with an unending age of harmony.
Receiving Thordans “blessing” means that his victims are “tempered” by him.
Once summoned, he is like other primals, able to “temper” whatever beeing he pleases. His victims develop a fanatical loyalty to the primal, creating a secure wellspring of faith. That likely happened to the Knights of the Heavens’ Ward.
Though there are 13 enemies the Singularity Reactor, only Thordan is the primal while his 12 knights are simply “blessed” by him.
According to Ser Grinnaux’s Lore entry for example, he was “blessed” by Thordan “to gain his loyalty once and for all”. So, we don’t know how long Grinnaux and/or the other members of the Heavens’ Ward were under the control of the primal. It is possible, they didn’t knew what they were doing all the time we were in Ishgard or fighting them in the singualarity reactor. If so Ser Zephirin didn’t even realized that he was going to kill the Warrior of Light.
The “blessing” would be the only real reason why men like Ser Haumeric the peaceful or like Ser Zephirin the Just who even carries his biggest character trait as his title were doing the things that happened in Heavensward.
They couldn’t escape the might of the primal Thordan, and even though they said “No” right from the beginning, the followers of Thordan VII likely would have killed the knights and lied about what happened to them, just like it happened to Zephirins predecessor. Or Thordan VII would have used them, like he already dies in Heavensward.
Remember what happened to Ser Vaindreau de Rouchemande? Thordan VII lied about his death too, saying he left Ishgard because a pilgrimage.