Vassal Hammer

The Vassal Hammer  or Seven Star Hammer, also known as the Hammer  Spirit of the Vassal Weapon. The spirit is from another world prior to being summoned to another world, the Vassal Hammer is one of the eight Vassal Weapons present in the world which Naofumi currently resides in. The candidates of the weapon have been given the title of Hammer Hero, whom they are generally addressed as so by the locals. The current candidate of the Vassal Hammer is unknown.

Overview
The Past Heavenly Emperor also known as Raphtalia's Ancestor, seems to have wielded a similar weapon in description of the Vassal Hammer, however, there's no solid proof whether the hammer that she has in her possession is the Vassal Hammer.

A few days after the four Legendary Heroes were summoned, the Hammer Hero went missing. It turns out that this Vassal Weapon wielder was killed and had the Vassal Hammer taken. The Vassal Hammer was next seen in Takt's possession during his campaign to take over Raphtalia's World. However, since he had collected all of the Vassal Weapons, he was unable to use any single one effectively. Eventually, Takt was defeated and Naofumi used his authority as a Holy Hero to strip Takt of his Vassal Weapons.

Power-up Method
The power-up method for the Vassal Hammer is "Weapon Form Synthesis". This method allows you to copy the most symbolic special effect from one form to another (ex. The Soul Eater Shield’s Soul Eat, the Rope Shield’s Rope), but the enhancement done to the source form is reset, reverting it to its original state. Each weapon has a given number of ability slots, but also a specific counter that informs you exactly how many more times that form can be used as synthesis material. Once this counter hits 0, the form is effectively locked out to power up methods. It can still be used as a weapon form, but is stuck at its base form and can’t be upgraded until you absorb the materials needed to unlock it again.

Trivia

 * In the Web Novel, Raphtalia wielded the Vassal Hammer.