Adamantine hammer

Grog Strongjaw briefly wielded a cursed adamantine hammer found in the tomb of the last dwarf king in the Crypts of Thomara. An attack made with the hammer that has a natural roll below a certain value is redirected against the attuned wielder. In the short time Grog had it, it was also nicknamed the Stop Hitting Yourself Hammer and Hammer of Face-Smash.

Description
The warhammer is intricate, though lacking runic detailing. It is made of adamantine, a rare and hard metal that is jet black in color. It is solid and hefty, and after a test swing, Grog determined it is one of the better hammers he's wielded.

If an attuned wielder makes an attack with the hammer and rolls a natural roll below ten, the attack is instead made against the wielder. The wielder takes the damage if the redirected attack succeeds against their armor class.

It is unclear if this hammer also has the typical properties of an adamantine weapon where it automatically deals critical hits to objects; properties for adamantine weapons in 5th edition were published in Xanathar's Guide To Everything in November 2017, three months after the episode this hammer appears in.

The hammer was cursed, described as there being "something wrong about its enchantment". The curse locked into Grog's soul, sinking in and occupying a portion of it. As a result, Grog was unable to part with the weapon, even to drop it.

History
The adamantine hammer was laid in the sarchopagus of the last dwarf king in the Crypts of Thomara. Vox Machina found it there while ascending Thomara in 812 PD and took it after Vax'ildan quickly destroyed the undead king. The hammer was given to Grog Strongjaw, who initially did not attune to it. However, Percival de Rolo's interest in the Cincture of the Stone King, also found in the sarcophagus, spurred Grog to attune to the hammer in place of the Bloodaxe as they continued walking through Thomara.

Grog shortly after used the hammer against undead trolls the party encountered, but his first attack caused him to strike himself in the face. He struck himself again later. Afterward, he was unable to let go of the weapon until its curse was lifted by Pike Trickfoot with Greater Restoration. He left it where it dropped.

Trivia

 * The hammer's properties are similar to javelins and spears of backbiting, which attempt to strike the attuned wielder if they roll a 1 on an attack roll; a new attack roll is made against the wielder's AC.