Insigne du Mage de Guerre

(requires attunement by a spellcaster)
This 1-inch-diameter gold coin allows you to use
a simple or martial weapon as a spellcasting focus
for your spells. For this property to work, you
must attach the coin to the weapon by pressing
the coin against it for at least 10 minutes. Thereafter, the coin can't be removed unless you detach it as an action or the weapon is destroyed.
Not even an Antimagic Field causes it to fall off.
The coin does fall off the weapon if your
attunement to the coin ends.