The Thoraxe is a large and long ancient scepter/axe found in The Beetle Lair shaped like an insect's abdomen, the thorax, with a red glow to it. It is seen in the inscriptions of a stone slab, in the hands of a mythical warrior demigod called the Death Knight, once worshipped within the Beetle Lair.
Cyanidium is a poisonous metal that drains the health of a kombatant. The Thoraxe, located in the Beetle Lair, is made of this same metal.
Baraka vs. Mileena
In the Baraka vs. Mileena video in Deception, once the fight was taken to the lower floor of the Beetle Lair, Baraka whom was knocked down into said floor decided to grab the nearby Thoraxe after getting up. However, in Mileena's ending, Baraka was killed despite trying to fight with it. (Although in Baraka's ending, it was revealed that it was just another Tarkatan, whom was sent out by Baraka to be his decoy.)
Cyanidium is a play on the poison Cyanide.