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 was stated in "The Chapter Closes" (8x16) that the hammer had been wielded and activated in the moonlight, it would gain 1 charge of greater invisibility.