Infernal Brimstone

Infernal Brimstone is a foul-smelling piece of demonic corestone mined off slain infernals. It is an incredibly rare ore.

According to Mama Diggs, it smells like the combined odors of sulfur and rotting corpses.

Source
The stone is obtained from the cores of fallen Brimstone Destroyers as part of associated world quests, available for miners. The core of the infernal does not disappear immediately after mining it, allowing other players to mine it before the infernal respawns.

Additionally it can be found inside Blingtron 6000 Gift Packages or in Salvaged Armor when using the salvager shoulder enchant.

Alchemy

 * Infernal Alchemist Stone

Blacksmithing

 * Demonsteel Breastplate
 * Demonsteel Greaves
 * Demonsteel Helm
 * Consecrated Spike
 * Flamespike
 * Gleaming Iron Spike
 * Terrorspike
 * Steelbound Harness

Enchanting

 * Enchanted Cauldron

Engineering

 * "The Felic"
 * Shockinator
 * Reaves Module: Wormhole Generator Mode
 * Blingtron's Circuit Design Tutorial
 * Ironsight Cranial Cannon
 * Trigger

Jewelcrafting

 * Saber's Eye
 * Ancient Maelstrom Amulet
 * Blessed Dawnlight Medallion
 * Grim Furystone Gorget
 * Intrepid Necklace of Prophecy
 * Raging Furystone Gorget
 * Righteous Dawnlight Medallion
 * Sorcerous Shadowruby Pendant
 * Subtle Shadowruby Pendant
 * Sylvan Maelstrom Amulet
 * Tranquil Necklace of Prophecy
 * Twisted Pandemonite Choker
 * Vindictive Pandemonite Choker
 * JewelCraft

As a quest objective
This item is an objective of the following quests:
 * Bindings of the Windlord (quest)
 * A Giant Ore Deal