Hero's Blade
Artifact — Equipment
Equipped creature gets +3/+2.
Whenever a legendary creature you control enters, you may attach this Equipment to it.
Equip 
(: Attach to target creature you control. Equip only as a sorcery.)
Russell Lu
This card will be released on 2025-06-13, it is not legal in any format until its release date approaches.
Rulings
If the legendary creature that entered the battlefield leaves the battlefield before Hero's Blade becomes attached, Hero's Blade stays as it was. If it's already attached to another creature, it remains attached to that creature.
(wotc • 2020-11-10)
The triggered ability will trigger when one of the Gods from Theros block enters the battlefield only if your devotion is high enough that it's a creature when it enters. If Hero's Blade is attached to a God that stops being a creature (or any creature that stops being a creature), it will become unattached.
(wotc • 2014-11-24)
Rulings
If the legendary creature that entered the battlefield leaves the battlefield before Hero's Blade becomes attached, Hero's Blade stays as it was. If it's already attached to another creature, it remains attached to that creature.
(wotc • 2020-11-10)
The triggered ability will trigger when one of the Gods from Theros block enters the battlefield only if your devotion is high enough that it's a creature when it enters. If Hero's Blade is attached to a God that stops being a creature (or any creature that stops being a creature), it will become unattached.
(wotc • 2014-11-24)