The Miracle Mallet is Shinmyoumaru Sukuna's belonging. Wielding it is her ability. Shinmyoumaru is a descendant of Issun-Boushi.

Shinmyoumaru did not know about the Miracle Mallet at first, because the inchlings had sealed it.

The Shining Needle Castle is a castle that was created by the Miracle Mallet. Apparently, it appeared from the desire to build a splendid castle to rule over the people.

The Miracle Mallet was originally an oni's tool. It can grant wishes, but it requires a cost―a common tale told in every world. Also, if the wish is too grand, you will be asked to pay up first...

Shinmyoumaru used the Miracle Mallet for revenge because of Seija, who had only approached Shinmyoumaru to convince her to wield the Miracle Mallet's power. Seija probably had not thought at the time that they would end up getting along so well.