The Thundrainium mace is a weapon used by Grune.
The mace is made of Thundrainium which he creates in The Ghost Warrior. The weapon has special properties that allows Grune to drain Jaga of his energy. It is able to defeat him even while using the Sword of Omens. Grune only lost due to the repercussions of his own treachery: after accepting Jaga's surrender he took the Sword of Omens, which destabilized his ghost form.