Imaginary Threats — ruling 1

No creatures that player controls will untap during their next untap step, even creatures that don’t attack. This includes creatures that enter the battlefield or become tapped after this spell resolves.

Imaginary Threats — ruling 2

If the opponent exerts any creatures they control, exert and the effect from Imaginary Threats stopping them from untapping both apply in the same untap step. Those creatures will untap as normal in the player’s subsequent untap step.

Imaginary Threats — ruling 3

If, during that player’s declare attackers step, a creature that player controls is tapped or is affected by a spell or ability that says it can’t attack, then it doesn’t attack. If there’s a cost associated with having a creature attack, its controller isn’t forced to pay that cost, so it doesn’t have to attack in that case either.

Imaginary Threats : règles, fonctionnement et interactions (MTG)

Imaginary Threats is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Creatures target opponent controls attack this turn if able. During that player's next untap step, creatures they control don't untap. Cycling (, Discard this card: Draw a card.)

How it works : Imaginary Threats

Original English text — Wizards of the Coast / Scryfall

#1July 14, 2017· Wizards of the Coast
No creatures that player controls will untap during their next untap step, even creatures that don’t attack. This includes creatures that enter the battlefield or become tapped after this spell resolves.
No other card is named in this ruling.
#2July 14, 2017· Wizards of the Coast
If the opponent exerts any creatures they control, exert and the effect from Imaginary Threats stopping them from untapping both apply in the same untap step. Those creatures will untap as normal in the player’s subsequent untap step.
No other card is named in this ruling.
#3July 14, 2017· Wizards of the Coast
If, during that player’s declare attackers step, a creature that player controls is tapped or is affected by a spell or ability that says it can’t attack, then it doesn’t attack. If there’s a cost associated with having a creature attack, its controller isn’t forced to pay that cost, so it doesn’t have to attack in that case either.
No other card is named in this ruling.