The primogenitor cannot been normally be kill any mean unless the primogenitor is killed the holy spear like yukina or using some special power even though I never other way other than using the holy spear that utilized the divine vibration

True but no primogenitor has been killed bay the spear so you can't tell even though they said it in the story right

Well actually they already are the original fourth primogenitor avrora florestina has been killed by the arrow that also a part of the original holy spear or at least her immortal body did die together with the root / the fourth primogenitor curse soul/ program that has been created by the tenbu / devas

No avrora mad kojou kill her becase she had rood back inside her and all the rest of the 12 famlliers as vell

Yes I understand but the fact that I want to says is that avrora make kojou kill her along with the root using key/arow that also the part of holy arrow. It might not show in the anime but in the volume 17 when yukina spear has broken reina pick the same arrow that killed avrora to use as the new core for the spear this only in the light novel which is a little different from the anime version where brought a new spear for yukina from the future