You can target a face-down monster for “Nightmare Wheel”.
If you activate “Nightmare Wheel” during the Battle Step and target a monster that is attacking, the attack stops.
“Nightmare Wheel” targets. You select the monster when “Nightmare Wheel” is activated.
“Nightmare Wheel” is destroyed when the targeted monster is destroyed, returned to the hand, removed from play, send to the Graveyard, etc.
“Nightmare Wheel” is NOT destroyed if the monster is flipped face-down. “Nightmare Wheel” continues to target the monster, inflict damage, and prevent the monster from attacking or changing battle position.
If a face-down monster targeted by the effect of “Nightmare Wheel” is rearranged by the effect of “Wandering Mummy” or “Shifting Shadows”, the monster’s controller must honestly announce which monster is the target of “Nightmare Wheel”.
The effect of “Nightmare Wheel” will be negated by “Royal Decree”, so you can attack or change the battle position of the monster. Also, if “Royal Decree” is active, and the monster is removed from the field, “Nightmare Wheel” remains on the field meaninglessly because its text that would destroy it is negated. If “Royal Decree” is later destroyed, the effects of “Nightmare Wheel” are not applied (such as the 500 damage) if there is no monster being targeted by “Nightmare Wheel”.
The effect of “Nightmare Wheel” is still applied if the targeted monster changes sides.
If the monster targeted by “Nightmare Wheel” becomes equipped to “Relinquished”, then “Nightmare Wheel” is destroyed.