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Deals damage and has a 100% chance to lower the target's Special Attack by one stage. Pokemon with the ability Soundproof are immune.
Snarl is not a very useful move; it is essentially a Dark-type Icy Wind that lowers Special Attack instead of Speed. While the Speed drop that Icy Wind provides is occasionally useful (especially in VGC play), a Special Attack drop is nowhere near as helpful. Try finding more useful support and attacking options before using Snarl; it usually isn't very hard.
Dark Pulse is a more powerful alternative.
Struggle Bug also lowers the opponent's Special Attack.