Of a sudden

Then of a sudden the rope was still—the stone had caught at the very edge. Gingerly the ape-man clambered up the frail rope.

Turning back, then, toward the basement staircase, she began to grope her way through blinding darkness, but had taken only a few uncertain steps when, of a sudden, she stopped short and for a little stood like a stricken thing, quite motionless save that she quaked to her very marrow in the grasp of a great and enervating fear.