Frightmare (Red)

Play this only if an Illusionist attack action card you control has been destroyed by phantasm this turn.

Phantasm

Rulings

From the Uprising release notes

You cannot play this card unless an Illusionist attack action card you control has been destroyed by phantasm this turn.
You cannot defend with Frightmare as it does not have the defense property.

Keyword notes

Phantasm - Phantasm is a new keyword that exists on Illusionist attacks. Illusionist attacks with phantasm are powerful but fragile. When an attack with phantasm is defended by a 6{p}+ non-Illusionist attack action card, the attack is destroyed and the combat chain closes.

If an attack action is destroyed, it does not resolve. Hit triggers and effects that require the action resolving such as go again do not resolve.
Both players have priority during the defending step. Note: This is a new rule introduced with the release of Monarch.
When an attack with phantasm is defended by a 6+ non-Illusionist attack action, a trigger to destroy the attack is put on the chain. Players can respond to this. When it resolves it checks if the attack action still fulfills the requirements to destroy the attack.
Phantasm only triggers when the game state changes from false to true. It does not trigger from true to true. (For example: if a defending card goes from 5 to 6 it will create a trigger, but a card going from 6 to 7 will not create another trigger.)