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Opposing Pokemon cannot switch out while the wielder is on the field, unless both Pokemon have Shadow Tag. Opposing Pokemon can escape by using Baton Pass, U-turn, or Volt Switch (as long as the wielder isn't immune to Volt Switch), or by holding the item Shed Shell. Shadow Tag does not prevent the effect of Eject Button. In multi battles, Shadow Tag does not affect your partner Pokemon, nor does it affect non-adjacent opponents.
Shadow Tag is still an awfully good ability, it's just that now Wobbuffet has had one of its main tools, Encore, become heavily nerfed in single battles, its relative value has reduced somewhat. Therefore, although Wobbuffet is still seen, you won't see it very often. However, do not underestimate this ability, as the power of trapping an opposing Pokemon especially with something threatening like Wobbuffet is not to be underestimated. Gothitelle, as well as Gothorita in the lower tiers, can make use of Shadow Tag from the Dream World to enable them to revenge kill almost anything, as well as potentially to set up Calm Mind, although they are typically too slow and weak to outright sweep teams. Even so, the worst Shadow Tag has to offer is yet to come, since Chandelure gets this as a Dream World ability, and has been proven to be horrible to face with the combination of Shadow Tag and its monstrous attacking power, not to mention the ability to freely set up Calm Mind. As far as abilities go, Shadow Tag is easily one of the best.
Found in Dream World