Amethyst of the Battle Mage

Etched with eldritch runes, this 1-inch-diameter Amethyst allows you to siphon ambient magic from a spell you cast into your weapon.

When you use your action to cast a spell, you may spend your bonus action to make a single attack with the attached weapon.

For this property to work, you must attach the Amethyst to the weapon by pressing the gem against it for at least 10 minutes. Thereafter, the Amethyst can’t be removed unless you detach it as an action, the weapon is destroyed, or your attunement to the it ends. Not even an antimagic field causes it to fall off. 

Notes: Extra bonus weapon attack after casting a spell, spellcaster