Stone of Flame

The Stone of Flame (also called the Stone of Fire) was one of three artifacts created by Belo'vir Salonar, a renowned mage, during the Troll Wars, along with the Stone of Light and the Stone of Spark. Belo'vir imbued it with the power of the Sunwell.

Eventually, the traitor Dar'Khan Drathir, raised into undeath once again, stole the stones and delivered them to the Scourge. Players are tasked with recovering them, and a weapon of their choice is imbued with their power.

Source
This item can be found at the top of the Howling Ziggurat (location 40,49) in a Worn Chest.

Stone of Flame as a quest objective
This item is an objective of The Twin Ziggurats.