It's in the bloodied room in the hidden place beneath the Altar in the Immaculate kind of Cathedral thing, where you kill those two huge demon (A Fire and a Poison one).
Void Aura :
They are invincible, you should split through them until you reach a huge Blood Stone. As stated by Zizzax when he told you about invading Void demons, only Tenebrium damage can hurt the huge Blood Stone.
