Pokémon Gold and Silver/Burned Tower

When you enter the Burned Tower, Ensuine will tell you that he has tracked a rare Pokémon,, to the Tower. Smash the rock in the northeast corner to get an Ultra Ball. Make your way into the center, where your Rival bitterly complains about not seeing Suicune, then attacks you with four Ghost-types. When you've defeated him, a hole will open under your feet and you'll fall to the lower level. Three Pokémon will awaken and fly away. Eusine informs you that these were the Legendary Pokémon; you'll encounter them again.