The Moonhammer is a moon-themed, two-handed warhammer crafted by Zahra Hydris as a gift for Grog Strongjaw, which she gave to him in "The Sunken Tomb" (4x06). Aside from testing the hammer and, due to his low intelligence, thinking it was far more powerful than it truly is, Grog has not used the Moonhammer in actual combat.
The Moonhammer's handle is made of dark metal that transitions into what looks like glistening ivory closer to the head. On the head are engravings in an unspecified language. Moon crescents are carved on the sides of the warhammer's head. It has been stated in episode 115 that the hammer should be wielded in the moonlight to use and/or activate.
The Moonhammer would gain 1 charge of superior invisibility while underneath the full moon.