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All Pokémon on the field faint after 3 turns. Even Pokemon that are in the invulnerable stage of Bounce, Dig, Dive, Fly, and Shadow Force are affected. If a Pokémon affected by Perish Song uses Baton Pass, whatever Pokémon is switched in will still be affected by Perish Song. The perish count will not be reset in that case. Switching a Pokémon out regularly or via U-turn will remove the effects of Perish Song from that Pokémon.
When using Baton Pass, the Perish Song count will be passed to the recipient.