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Deals damage, but the user faints after its use. If the target uses Protect, is a Ghost-type, or the move misses, then the move will deal no damage, but the user will faint regardless.
Selfdestruct never was a good move due to the sheer existence of Explosion. In fact, the only Pokemon that have access to Selfdestruct but not Explosion are Snorlax, Munchlax, Wailord, and Mewtwo. To make things worse for Selfdestruct, BW mechanics makes Selfdestruct and Explosion almost laughable moves due to the severe drop in power. So in short, don't use Selfdestruct in BW, even with Snorlax or Munchlax.
Explosion is an equivalent with higher Base Power.