The question pertains to Annie Le Brun's Soudain un bloc d'abîme, Sade ("suddenly a block of abyss, Sade", published in English translation as Sade: A Sudden Abyss). What justifies its title?
Le Brun writes:
But since everything begins with The One hundred and Twenty Days of Sodom and since Sade has chosen to to begin with something that defies all justification, I shall begin by recalling this work – and recalling it insistently.
For it is with this text […] that there first appears that stupefying crystallisation of the blackest possible perspectives which constitutes the universe of Sade. Everything that Sade would subsequently write has elements of this, although nothing previously predicted the appearance, in the mid-Enlightenment, of this block of abyss, even in the intellectual trajectory of the rebellious young Marquis.
We do know that in 1785, Sade had already been in prison for seven years – and for the second time – for reasons of “extreme debauch.” We also know that, as early as 1782, he had already given violent expression and firm exposition to his atheistic views in his Dialogue between a Priest and a Dying Man (Dialogue entre un prêtre et un moribund). But none of this can adequately explain the sudden advent of a world which would henceforth disrupt the field of consciousness, and disrupt it absolutely. Nor does it explain the extraordinary discretion of Sade’s critics as to the significance of this event. Maurice Heine mentions it , of course; so does Gilbert Lely. They mention it more often, and much better than the others. But as one gets to know Sade’s thought as one sees it developing – if only chronologically around the mysterious kernel of The One Hundred and Twenty Days, one cannot help but wonder about the reluctance shown by all his critics when evaluating the real importance of this text in the overall body of his work.
Our horror at reading The One Hundred and Twenty Days of Sodom further derived from a dizzying derealization accomplished at the cost of an implacable objectification, this objectification in turn being accomplished at the cost of an absolute derealization. Suddenly, we have nothing left to hold onto, not even the uncertain parameters of self-awareness, which has disappeared unnoticed. Sade hurls us into the abyss we naively thought existed between the real and the imaginary, but which turns out to be the unbearable infinity of freedom.
This abyss is a space which fills all space, and from which we cannot ever emerge, since it consists of all that is real being negated by the imaginary, and all that is imaginary, negated by the real. It is an infinite space from which we can never withdraw since we are perpetually present at its mobile center. It is a space infinitely enclosed, or it is an infinity consisting of closed spaces. Either way it is a space into which Sade will never now stop dragging us.
So, it is clear that the ‘block of abyss’ is not Sade himself, even if he is the route by which we reach the abyss. The ‘block of abyss’ is the work The One Hundred and Twenty Days of Sodom. From these quotes it seems that Le Bruin does not regard The One Hundred and Twenty Days of Sodom as ‘the abyss’ itself, but as an essential artefact by which Sade conveys is to the abyss itself, it is a representation of the whole.
Note that the English translation of Le Brun’s book is called Sade: a Sudden Abyss, while the French original is titled Soudain un bloc d'abîme. In conflating the two titles into Sade: a Sudden Block of Abyss you have created an identification of Sade as the block of abyss which does not precisely appear in either the original title or that if the translation.
Edit Note: At the time I began answering the question, it read:
Why Annie Le Brun said that "Marquis De Sade" was a "Sudden block of Abyss "?
Annie Le Brun written a book called Sade A Sudden block of Abyss.
This is why the focus of my answer is on the abyss and Sade not being the same thing.