Obsidian Stone of War

Etched with eldritch runes, this 1-inch-diameter obsidian 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 obsidian to the weapon by pressing the obsidian against it for at least 10 minutes. Thereafter, the obsidian 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 obsidian does fall off the weapon if your attunement to the obsidian ends.