Phantasmaclasm (Red)

When this attacks, look at the defending hero's hand and choose a card. They put it on the bottom of their deck then draw a card.

Phantasm

Rulings

From the Monarch release notes

You must select a card from their hand. It is not optional.
If the defending hero’s hand is empty, they will still draw a card when Phantasmaclasm’s ability resolves.
Phantasmaclasm’s ability happens before the defending player has a chance to defend or play defense reactions. (And thus before it can be destroyed by phantasm)
If their deck is empty, Phantasmaclasm would put the chosen card on their deck and that player would immediately redraw the same card.
If this is attacking an ally, you would not look at a hero’s hand as there is no defending hero and thus no one would draw a card.

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.)