The phrase that most establishes tension is "as though he waited for some beast of the woods to come within the range of his vision."
This phrase creates a sense of anticipation and unease. The imagery of waiting for a "beast" suggests that something dangerous or threatening could emerge from the forest at any moment. It implies a feeling of vulnerability and highlights a hostile environment, both of which contribute to the overall tension of the scene. The context of the dark forest and the solitary watch also enhances this feeling of suspense and danger, making it clear that the character is on edge and anticipating an imminent conflict or encounter.