The ultimate bedrock of all division is Falsehood just as the ultimate ground of unity is Truth. It is falsehood that gives a lie to the Divine Reality and, even if it admits and imitates it, it declares an unbridgable gulf between the Divine Reality and the World. Falsehood is the last inevitable result of Ignorance. If Death were the last result then Life would be the original Divine Principle.
At one point the Mother herself contemplates this question but later answers that it is indeed Falsehood. The reason why Sri Aurobindo mentions Death in Savitri as the final opponent is because such is the framework of the story. But all the description of the being of Death is that of Falsehood which he also describes in Savitri Book Two Canto Eight as the home of evil. Of course the two Falsehood and Death are close cousin and their first offspring fear.
Affectionately,
Alok Da
