The die will inflict a status affliction (sleep, paralysis, etc.) on the opponent, and it can be used once per battle.
For your Mystery Pokemon game: Huntail, Mothim, Tentacool, Ekans, Totodile, Smeargle, Dunsparce, Dratini, Natu, Wurmple, Tropius, Slowpoke, Horsea, Probopass