Herald of Sekem (Red)

When this attacks, you may put a yellow card from your hand into your soul. If you do, deal 2 arcane damage to any target.

Phantasm

Rulings

From the High Seas release notes · June 6 2025

Herald of Sekem triggered when it resolves and becomes attacking. This is an optional effect: Because it has a conditional targeted effect, you must declare a target when you add this triggered effect to the stack. You choose whether or not to put a yellow card from your hand into your soul at the time the triggered effect resolves. If you do put a yellow card from your hand into your soul this way, you will deal 2 arcane damage to that target.

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.)