Boosts the power of the Pokémon's Bug-type moves by 50% when its HP drops to 1/3 or less of its max.
Swarm (Japanese: むしのしらせBug Notification) is an Ability introduced in Generation III.
Effect
In battle
Generations III to IV
When a Pokémon with Swarm uses a Bug-typemove, the move's power will be increased by 50% if the user has less than or equal to ⅓ of its maximum HP remaining.
Generations V onward
Instead of boosting Bug-type moves' power, Swarm now technically boosts the Pokémon's Attack or Special Attack by 50% during damage calculation if a Bug-type move is being used, resulting in effectively the same effect.
The Japanese name for Swarm, むしのしらせ Mushi no Shirase (Bug Notification), is a Japansese expression that translates to "foreboding; premonition; gut feeling". This is reflected by the way the Ability works, as the Pokémon is preparing itself for a tough fight by making itself stronger.
Swarm was the first Ability in animation to be a Pokémon's confirmed Ability without getting activated.