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This move functions differently for Ghost-type Pokemon. If the user is a Ghost-type Pokemon it will put a curse on the target, dealing 1/4 of the foe's max HP to the target at the end of every turn. However, it also damages the user by 1/2 of its max HP when used. If the target has the ability Magic Guard, it will not be affected by the curse. This move cannot be blocked by Protect or Detect. This move cannot be blocked by Substitute. If the target uses Baton Pass, the replacement will continue to be affected by the curse. Fails if there is no target or if the target is already affected.
When used by a Pokemon that is not a Ghost-type, it will boost the user's Attack and Defense by one stage and lower the user's Speed by one stage. If used by a Ghost-type with Magic Guard this last effect also applies. However, it will fail if there is no target, despite working as a boosting move.
This move has very different effects depending on what type of Pokemon uses it. Generally speaking, it's not good if used by a Ghost-type, since the target can simply switch out, negating the effects completely, not to mention that half your HP is a high price to pay for the effect. On the other hand, it can be useful on full stall teams that have problems dealing with that last Pokemon that is immune to Toxic, who can otherwise boost without fear of being forced out by phazing moves, and sweep the team, such as Metagross or Lucario. However, if Curse is used, it can create a "win condition" considering that the foe can't switch out to nullify the effects, and apart from Magic Guard users nothing is immune to it. A good user of this technique is Spiritomb. However, Perish Song is a far better alternative if you really need a technique of this sort.
From a non-Ghost point of view, Curse is a boosting move that trades Speed for more Attack and Defense. Good candidates, therefore, are bulky Pokemon that simply don't care about Speed and invest fully in HP and Special Defense, letting Curse boost the rest. Unfortunately, due to the very fast-paced, offensive nature of most of the tiers, Curse is rarely seen in competitive play. Snorlax is certainly the most famous Curse user in the game, though its Curse sets are far less effective in the fifth generation OU metagame. However, CurseLax is still a very real threat in UU. Ferrothorn is about the only noteworthy user of Curse in OU, as not only does Curse boost its good Attack and Defense stats, but it also lowers its Speed, powering up its STAB Gyro Ball against slower threats.