gistfile1.txt

// target is the player
// transform is the spider

Vector3 orientation = GetTerrainNormal();
Vector3 directionToTarget = (target.position - transform.position).Y(0);
float d = Vector3.Dot(directionToTarget, orientation);
directionToTarget -= d * orientation;
if (directionToTarget.sqrMagnitude > 0.00001f) {
  directionToTarget.Normalize();
  Quaternion rotationNeeded = Quaternion.LookRotation(directionToTarget, orientation);

  transform.rotation = Quaternion.RotateTowards(
      transform.rotation,
      rotationNeeded,
      xRotationSpeed * Time.deltaTime
  );
}