Weapon as Familiar

Fighting your enemies can be hard work sometimes, and they're usually far from worth the trouble, especially if you've already been injured beforehand. Solution - your Empathic Weapon will take over or give a boost to help you out.

There are three different scenarios: the first is a change in form, giving an additional set of powers to your Empathic Weapon. The second is to turn your Empathic Weapon into an effect, like smoke or energy. The third is when your Empathic Weapon becomes an actual creature that fights for you, independently or otherwise.

Inverse of One-Winged Angel - your weapon did the transformation in your place. Compare Guardian Entity. Also compare Living Weapon, Equippable Ally and the slightly more mundane Attack Animal.

Anime and Manga

 * Bleach's Zanpakuto are essentially a splinter of their wielder's soul in the form of a sword that acts as the vessel of their powers. There are examples of all three types, though the third is the least common.
 * In Yaiba, during the Pyramid Arc, he's on the receiving end of a Curb Stomp Battle from Diamond, who can predict and stop all his attacks. In order to turn the table, he let Fujin and Raijin, the demons living in his swords, possess him, so that they can trash Diamond around with their combined powers.

Tabletop Games

 * The Singing Sword from Munchkin, which dances after you on legs of its own, counts towards your attack damage, and sings.
 * There's also the Two-Handed Sword, which has two hands of its own and is shown on the card wielding smaller swords.

Video Games

 * The Sword Familiar in Castlevania: Symphony of the Night, though it's more of a creature that fights for you that you can equip as a weapon.
 * In Musashi: Samurai Legend, during the final boss fight Gandrake
 * Four examples in Soundscape, all of which come from the Precursors.
 * Dolecrel is the template for the 'effects' scenario, including the elements. Scimitars based off him double as a Technicolor Blade.
 * Tostrow is the template for the animal-based Scimitars.
 * Sherruk, template for the armor-based Scimitars. Here, the Empathic Weapon is your armor, which is outfitted with an arsenal of abilities, placed in three categories - defense, support and assault.

Web Comics

 * Sluggy Freelance has Torg's sword, Unholy Evil Death Bringer, powered by the blood of the innocent, AKA Weeping God, AKA Chaz, is actually a fairly decent fellow.