Perish Song is an offensive Normal-type move.
All Pokémon on the field when Perish Song is used gain a perish count, except Pokémon with the Ability Soundproof and Pokémon that already have a perish count. Starting at 4 on the turn Perish Song is used, the perish count decreases by 1 at the end of each turn (including the turn Perish Song is used in). When a Pokémon's perish count reaches 0, it faints.
If an affected Pokémon switches out with Baton Pass, the perish count will be transferred to the Pokémon switched in, even if it has the Ability Soundproof. If it switches out in any other way, neither it nor the switched in Pokémon will have a perish count.
Pokémon that learn Perish Song
By leveling up
Via a prior evolution