Cryptogram from Frankenstein; or, the modern prometheus, chapter 20

I trembled and my heart failed within me, when, on looking up, I saw by the light of the moon the dæmon at the casement. A ghastly grin wrinkled his lips as he gazed on me, where I sat fulfilling the task which he had allotted to me. Yes, he had followed me in my travels; he had loitered in forests, hid himself in caves, or taken refuge in wide and desert heaths; and he now came to mark my progress and claim the fulfilment of my promise.

The sentence you are solving goes here.

I left the room, and locking the door, made a solemn vow in my own heart never to resume my labours; and then, with trembling steps, I sought my own apartment. I was alone; none were near me to dissipate the gloom and relieve me from the sickening oppression of the most terrible reveries.