The most fitting word to replace "dark" in a way that creates a greater sense of dread is "shadowy."
Explanation: "Shadowy" evokes imagery of obscurity and uncertainty, which can enhance feelings of fear and suspense, especially in the context of a prowling threat in the forest. "Opaque" and "ebony" may convey darkness, but they lack the same connotations of danger and the unknown that "shadowy" embodies. "Dim," while it suggests low light, does not inherently carry the same ominous tone.