Demostración de la mecánica: https://imgur.com/UNaAm9s

Scripts:

Se define en PBTerrains los tags isLuz? y isSombra?
Se define F como input en Controls
Se definen las formas en MultipleForms

---En Scene Map---
    #Cambio de forma 802
    if Input.trigger?(Input::L) && !pbMapInterpreterRunning? && $game_switches[101]
          for i in $Trainer.party
            if i.species==1
              if  $game_variables[101]==1 #si es luz, pasa a sombra
               if PBTerrain.isLuz?(pbGetTerrainTag)
                 Kernel.pbMessage(_INTL("¡No puedes transformarte aquí!"))
                 break
               end
               i.form = 0
               $game_variables[101]=0
             else #si es sombra, pasa a luz
                if PBTerrain.isSombra?(pbGetTerrainTag)
                   Kernel.pbMessage(_INTL("¡No puedes transformarte aquí!"))
                   break
                end
                i.form = 1
                $game_variables[101]=1
              end
            $game_map.refresh
            pbSEPlay("expfull")
            break
          end
         end
        end


----En Game_Map----
  #Sombra y luz comprobaciones
      elsif PBTerrain.isLuz?(@terrain_tags[tile_id]) && $globalluz==0
        Kernel.pbStartOver
      elsif PBTerrain.isSombra?(@terrain_tags[tile_id]) && $globalsombra==0
        Kernel.pbStartOver
      elsif PBTerrain.isLuz?(@terrain_tags[tile_id]) && $game_variables[101]==0
          return false
      elsif PBTerrain.isSombra?(@terrain_tags[tile_id]) && $game_variables[101]==1
          return false

------En PBField_Field, bajo field_movement------
$globalluz=250
$globalsombra=250

Events.onStepTaken+=proc{
  if PBTerrain.isLuz?(pbGetTerrainTag)
    $globalluz-=5
  elsif PBTerrain.isSombra?(pbGetTerrainTag)
    $globalsombra-=5
  end
}

--------Sobre Main-----------
class Spriteset_Map
  alias old_initialize initialize
  def initialize(map=nil)
    @bar=IconSprite.new(0,320,@@viewport1)
    @bar.bitmap=Bitmap.new(Graphics.width,Graphics.height)
    @colors=[
        Color.new(255,255,255),Color.new(160,160,160),     # Luz
        Color.new(0,0,0),Color.new(160,160,160),      # Sombra
     ]
    old_initialize(map)
   end

   alias old_update update
   def update
     @bar.bitmap.clear
     if $game_switches[101]==true
      #Luz
      @bar.bitmap.fill_rect(Rect.new(10,40,250*10/20,6),@colors[1])
      @bar.bitmap.fill_rect(Rect.new(10,40,$globalluz*10/20,6),@colors[0])
     end
      #Sombra
      @bar.bitmap.fill_rect(Rect.new(10,50,250*10/20,6),@colors[3])
      @bar.bitmap.fill_rect(Rect.new(10,50,$globalsombra*10/20,6),@colors[2])
     @bar.z = 1000
      old_update
   end
 end