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The Miracle Mallet (打ち出の小槌 uchide no kozuchi) is a legendary artefact from the tale of Issun-bōshi, a small hammer with the power to change an object's size. The diminutive protagonist steals the mallet from an oni, using its power to grow to the size of a normal human.
In the Touhou Project, the mallet's powers function only for members of Issun-bōshi's race, the inchlings, with its primary wielder being Shinmyoumaru Sukuna. In addition to size-changing, it can grant the wishes of the wielder, but at a cost proportional to the scale of the wish.
The energy emitted by the Miracle Mallet has the power to create tsukumogami, though this effect seems to be only partially under the user's control. Some time prior to the events of Hopeless Masquerade, this power began "corrupting" Reimu Hakurei's purification rod and Marisa Kirisame's Mini-Hakkero, increasing their power but making them harder to control. By the time of Double Dealing Character, these weapons, along with some of Sakuya Izayoi's knives, were capable of moving around on their own. After the resolution of the incident Shinmyoumaru began withdrawing these energies back into the Mallet, and by the time of Forbidden Scrollery chapter 12, the weapons have completely returned to normal. Some newborn tsukumogami however, including Benben, Yatsuhashi and Raiko, found alternate sources of power that allowed them to remain in existence.
The Mallet's energies also have mind-influencing effects, with a number of normally peaceful youkai going berserk during its appearance. After the events of Double Dealing Character, it was realised that tsukumogami created by the Mallet were unusually passive towards their human owners. Likewise, their owners became more youkai-like in behaviour, and oddly unconcerned with their tools' strange new properties.
In Chapter 5 of Eastern and Little Nature Deity, one of the Great Tengu sends the three fairies of light a tool that greatly resembles the Miracle Mallet, and that was called a "Tengu treasure" (but was only used to break a giant egg). Another similar tool is also present in the main screen of Undefined Fantastic Object, inside the boat ridden by Reimu. It is unknown if there is any relation between the true Miracle Mallet and those other two mallets, or if it is just a coincidence.