Here's how it works:
1. Activate "Call of the Haunted" during your opponent's Main Phase 1 or Main Phase 2.
2. Choose one of your opponent's monsters in the graveyard and Special Summon it to your side of the field.
3. Your opponent chooses a card from their hand. They don't show you what it is.
4. When the Special Summoned monster you control attacks, your opponent guesses what card they chose.
5. If they guess correctly, the Special Summoned monster gains 1000 ATK.
6. If they guess incorrectly, the Special Summoned monster loses 1000 ATK.
Note: The effect of "Call of the Haunted" only applies when the Special Summoned monster attacks a monster in defense position.