The Defending Pokémon can't retreat as long as Murkrow remains your Active Pokémon. (Benching or evolving either Pokémon ends this effect.)
Choose 1 of your opponent's Pokémon. This attack does 20 damage to that Pokémon. This attack's damage isn't affected by Weakness, Resistance, Pokémon Powers, or any other effects on the Defending Pokémon.