Dragon Hammer is a combination of the 10x Kamehameha and Dragon Fist used by Goku in his Super Saiyan 4 form. A technique also used with a combination of the God Kamehameha and Super God Fist used by Goku in his Super Saiyan Blue Kaio-ken form.
Goku fires the 10x Kamehameha directly at the opponent's chest. Then, while the beam is blasting the enemy, Goku flies through the attack and punches the opponent in their stomach with the Dragon Fist. Finally, Goku charges more power into the attack, resulting in the golden dragon overpowering the opponent as it coils around them and pierces through their stomach, inflicting a huge amount of damage.
Goku uses this attack during his battle against Omega Shenron. After Goku recovers his sense of sight, Omega Shenron still believes Goku is blind and even taunts him by allowing him to attack. At that moment, Goku fools Omega Shenron by suddenly opening his eyes and attacking him with the Dragon Hammer, seemingly destroying the Shadow Dragon. However, the victory is short-lived, as Omega Shenron survives by regenerating himself using Rage Shenron's technique, Electric Slime Body Built and continues the fight.
In Dragon Ball: Yo! Son Goku and His Friends Return!!, Goku uses a similar attack to defeat Aka (although the golden dragon at the end of the attack is not present).
In the Dragon Ball Super anime, Goku fires a fully charged 10x God Kamehameha at his opponent and then charges through the Kamehameha using a Super God Fist to end the attack off. Goku uses this attack against Hit but Hit uses Time-Skip to dodge it.
Video Game Appearances
In Dragon Ball FighterZ, it appears as a Meteor Attack used by Super Saiyan Blue Goku under the name Evolved Attack which can be performed after SSGSS Goku's x10 Kaioken Kamehameha Meteor Attack. The Evolved Attack portion consumes two ki gauges in addition to the three ki gauges required to perform the x10 Kaioken Kamehameha (thus both attacks together cost five ki gauges). When Evolved Attack is performed, Goku will charge through his x10 Kaioken Kamehameha and punch his fist through the opponent which generates an explosion that damages the helpless opponent.