Spectral Rider (Yellow)

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When this is played, if you control a Spectral Shield, this gets overpower.

Phantasm

Rulings

From the Dynasty release notes

The condition of controlling a Spectral Shield is evaluated after Spectral Rider is played but before it becomes attacking.
If you create a Spectral Shield after Spectral Rider is played, but before the trigger resolves, Spectral Rider will gain overpower.
If you create a Spectral Shield after the trigger resolves, Spectral Rider does not retroactively gain overpower.

Keyword notes

Overpower - The defending hero can’t defend this with more than one action card.

Overpower is a static ability that generates a continuous effect.
A player can not declare more than one defending action card against an attack with overpower.
Overpower does not prevent additional action cards being added as defending cards through effects (e.g. Amulet of Havencall).

All keywords

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.)