Herald of Rebirth (Blue)

When this hits, put it into your hero's soul and put up to 1 card with phantasm from your graveyard on top of your deck

Phantasm

Rulings

From the Monarch release notes

If the attack is put into your hero’s soul, the combat chain does not close and the chain link remains on the combat chain along with all of the defending cards and attack reactions.
If the attack is put into your hero’s soul, Phantasm is no longer functional as a triggered ability, and the attack can not gain abilities such as “go again”.
If the attack has had “go again” before it goes into your soul, Last Known Information is used at the end of chain link resolution and you will still gain an action point.
You can only put a card with the keyword phantasm from your graveyard on top of your deck, this does not include cards that only reference phantasm.
Herald of Rebirth cannot put itself to the top of your deck as it is not in the graveyard when it hits.
You do not have to put a card with phantasm from your graveyard on top of your deck. Even if you elect not to, you must still put Herald of Rebirth into your hero’s soul.
If this attack hits, you must put it into your hero’s soul. This effect is not optional.

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.)