The Black Diamond Pike was a unique weapon Connor obtained during his quest.
While in the Barren Region, the Weirdling Tradesman informed Connor of the Basilisk terrorizing the land. Connor offered to slay the Basilisk and the tradesman told Connor to get him a black diamond and a strong metal shaft and he could craft for him a black diamond pike since only a black diamond could pierce the Basilisk's thick hide. Connor agreed and after retrieving a shaft of magical metal from the nearby blacksmith, gave both it and the Dragon Wyrm's black diamond heart to the tradesman. After Connor handed over ten gold coins and his old hand weapon, the tradesman crafted the black diamond pike for him, telling Connor that the Basilisk's only vulnerable spot is right behind its head.
Connor manages to sneak up on the Basilisk by climbing the cliff behind it from the other side. Once on top of the cliff, Connor jumps on top of the Basilisk and rams the black diamond pike through its vulnerable spot, killing the Basilisk. As the Basilisk falls, Connor twists the pike in its wound to increase its effectiveness. After the Basilisk's death, Connor finds himself unable to remove the black diamond pike from its corpse, the pike having gotten stuck. Connor notes that the black diamond pike is of no more use to him anyways and abandons the weapon in the Basilisk's corpse.