Hero's Blade

Equipped creature gets +3/+2.
Whenever a legendary creature you control enters, you may attach Hero's Blade to it.
Equip {4} ({4}: Attach to target creature you control. Equip only as a sorcery.)

Doctor Who

The best swords are forged with dragonfire.

Commander Masters

Equipped creature gets +3/+2.
Whenever a legendary creature enters the battlefield under your control, you may attach Hero's Blade to it.
Equip {4}

Jumpstart 2022

Equipped creature gets +3/+2.
Whenever a legendary creature enters the battlefield under your control, you may attach Hero's Blade to it.
Equip {4} ({4}: Attach to target creature you control. Equip only as a sorcery.)

Dominaria United Commander

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.

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.