Magic is available by default to blacksmith Rickert of Vinheim, who can be found early in the New Londo Ruins. Initially, he can only ascend Magic weapons to +5, but he later gains the ability to ascend to +6 when given the Large Magic Ember from The Duke's Archives.
To create a Magic weapon, players need a +5 standard weapon, Green Titanite Shards (from +0 to +5), Blue Titanite Chunks (from +6 to +9), and a Blue Titanite Slab (for +10), as well as the requisite amount of souls.
By upgrading a weapon down the Magic upgrade path, it gains increased Magic Damage scaling through Intelligence, while also losing Physical Damage through Strength and Dexterity scaling. This makes the Magic path suitable for those with a high Intelligence stat, but low Strength and Dexterity stats.
A standard weapon with a magical weapon augmentation, such as Crystal Magic Weapon, deals more damage than Magic weapons in almost every instance; however, Magic weapons have the advantage of being permanent and not having to be buffed.
The Enchanted upgrade path offers even better scaling with Intelligence than the Magic path, albeit at the cost of even heavier Strength and Dexterity bonus reductions.