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Deals damage at the end of the turn after three turns.
The damage is based on the Special Attack and Special Defense at the time of using Future Sight, not at the time of striking. This means that if Future Sight is used on a Pokémon with 100 Special Defense, and then the opponent switches to a Pokémon with 300 Special Defense, the damage calculations will be done as though the target has 100 Special Defense.
Future Sight is technically typeless, so it does not factor in STAB, and hits all Pokémon neutrally. Because it damages at the end of the turn, it is not blocked by Detect or Protect, and can KO a Pokémon that used Endure. If the target uses Detect or Protect on the turn Future Sight is selected, however, that will count as a successful Detect or Protect against Future Sight.
Multiple Future Sights cannot be queued for the same Pokémon. Future Sight must hit the target before it can target that Pokémon again.