Flip a coin. If heads, the Defending Pokémon can't retreat. (Benching or evolving that Pokémon ends this effect.)
If this attack damages the Defending Pokémon, the Defending Pokémon is now Poisoned and you remove a number of damage counters from Ariados equal to half that damage (rounded up to the nearest 10). If Ariados has fewer damage counters than that, remove all of them.