The Lucky Mallet is a divine tool that can turn anything or anyone, smaller, or larger. Also, it can create gold and wealth from thin air. It belongs to one of the Seven Lucky Gods, Daikokuten.
It was borrowed by Narukami in an failed attempt to make Tomoe her familiar. Afterwards it was given to Nanami who turned Tomoe back to normal then used by the will-o-wisps to make themselves bigger. It was forgotten in Mikage shrine until Onikiri and Kotetsu found it and played with it, while Nanami was in Izumo. Their playtime was interrupted by a message from Otohiko and Tomoe turned them back to normal. When Tomoe rushed to Yomotsu Hirasaka, Kotetsu clung to Tomoe with the Mallet stuck in his belt.
He used it to turn Tomoe back into a yokai capable of defeating Ikusagami, the God of War, who was sitting on the giant seal of the Yomi-no-Kuni entrance. After Nanami was rescued and carried back to Ookuninushi's shrine, the host himself paid a visit to Nanami with the Mallet in his possession. Ookuninushi explained that since Tomoe used the hammer to turn himself into a yokai (whatever the reason may be) he committed a sinful crime which eventually led to his imprisonment.