The black stone or black prism is an object located in Zemouregal's fort in The North during the quest Ritual of the Mahjarrat. It is a magical item created from Obsidian which its owner, Zemouregal, created using materials he found in the Karamja Volcano.
According to Zemouregal in his Heart magic notes, it will give him the ability to have greater control over the undead hero Arrav's heart, making him less likely to resist Zemouregal's powers.
During the quest, the player infiltrates the fort and smashes the stone, allowing Arrav to be swayed when he comes into contact with his heart.
When examining it a message will appear: "A destructive hooligan has vandalised this!"