The Mace of Ulumak allows Gor-Rok to re-roll all failed To Hit rolls. In addition, when striking with the mace of Ulumak, Gor-Rok gains an additional Attack from his Predatory Fighter special rule in any successful To Hit roll of a 5 or 6.
Source: Warhammer Fantasy: 8th Edition
The Mace of Ulumak allows Gor-Rok to re-roll all failed To Hit rolls. In addition, when striking with the mace of Ulumak, Gor-Rok gains an additional Attack from his Predatory Fighter special rule in any successful To Hit roll of a 5 or 6.