How could I use bent normal map
Here is an interesting answer from Styves on gamedev.net.


  Calculating bent normals is just an extension of AO calculation where
  the direction of each sample is also averaged along with the occlusion
  amount.
  
  
  
  The bent normals from substance painter seem correct. In any region
  that there is no AO, your normal won't be bent in any direction,
  therefore in tangent space it's pointing straight up along the surface
  normal. This is why you only see details in the ears.
  
  I have no idea how Unity handles the bent normals but the usual way is
  to, instead of applying AO as a standard multiplier onto the lighting
  result (which gives this weird muddy look), use the dot product of the
  bent normal and the light instead. This is how we handle SSDO in
  CryENGINE for example.
  
  I've only ever used a texture-based version of this technique once,
  however in this version the bent normals also contained information
  from the original tangent space normal map, and was used directly
  during lighting in place of the original.