Pokémon HeartGold Version · Pokémon SoulSilver Version
In SoulSilver, use the Silver Wing and Tidal Bell to meet Lugia in the Whirl Islands. In HeartGold, use the Rainbow Wing and Clear Bell to meet Ho-Oh in the Bell Tower.
The other legendary becomes available later. Get its wing from the man on the hill in Pewter City.
Lugia stays in the Whirl Islands and Ho-Oh stays in the Bell Tower. The later encounter is level 70.
Finish the main story. For Xerneas, complete Mable’s unusual Alpha Pokémon tasks in Wild Zones 3, 5 and 9. Xerneas then appears in Wild Zone 11.
For Yveltal, beat three stronger Rogue Mega Pokémon in the Quasartico simulations. Then find Yveltal on the roof of Galerie de la Lune in Rouge Sector 2.
Catch Xerneas and Yveltal and defeat L. Then follow Zygarde to Prism Tower for its multi-phase battle.
How do I find the Paradox Pokémon in Perrin's photos?Scarlet and Violet · The Indigo Disk · Spoilers
Finish Perrin's Bloodmoon quest and register 200 Pokémon in the Blueberry Pokédex. Her photos point to locations in Area Zero.
Talk to Perrin in the Terarium after meeting both requirements. She will show you two photos.
In Scarlet, follow the stream upstream from Research Station 3 to the waterfall for Raging Bolt. Gouging Fire stands by the satellite dish above the waterfall across from Zero Gate.
In Violet, find Iron Crown on the cliff edge above Research Station 1. Look below Station 2 behind the slanted rock slab for Iron Boulder.