Cryptogram from Frankenstein; or, the modern prometheus, chapter 21

Yet one duty remained to me, the recollection of which finally triumphed over my selfish despair. It was necessary that I should return without delay to Geneva, there to watch over the lives of those I so fondly loved and to lie in wait for the murderer, that if any chance led me to the place of his concealment, or if he dared again to blast me by his presence, I might, with unfailing aim, put an end to the existence of the monstrous image which I had endued with the mockery of a soul still more monstrous. My father still desired to delay our departure, fearful that I could not sustain the fatigues of a journey, for I was a shattered wreck - the shadow of a human being. My strength was gone. I was a mere skeleton, and fever night and day preyed upon my wasted frame.

The sentence you are solving goes here.

Ever since my recovery from the fever, I had been in the custom of taking every night a small quantity of laudanum, for it was by means of this drug only that I was enabled to gain the rest necessary for the preservation of life. Oppressed by the recollection of my various misfortunes, I now swallowed double my usual quantity and soon slept profoundly. But sleep did not afford me respite from thought and misery; my dreams presented a thousand objects that scared me. Towards morning I was possessed by a kind of nightmare; I felt the fiend's grasp in my neck and could not free myself from it; groans and cries rang in my ears. My father, who was watching over me, perceiving my restlessness, awoke me; the dashing waves were around, the cloudy sky above, the fiend was not here: a sense of security, a feeling that a truce was established between the present hour and the irresistible, disastrous future imparted to me a kind of calm forgetfulness, of which the human mind is by its structure peculiarly susceptible.