Where do you find the forbidden song in Pokemon ranger guardian signs?

In Pokémon Ranger: Guardian Signs, the Forbidden Song can be found in the area known as the "Altru Building." To obtain it, you'll need to progress through the story until you reach the appropriate point where the song is revealed. Players must complete certain missions and interact with specific characters to access the song. Once you have it, you can use it to summon various Legendary Pokémon during your adventures.