Arcane Horror Heart is one of the Rune Crafting Materials in Dragon Age: Inquisition (DAI). Once the Schematic for a rune has been acquired, the player can produce Runes of that quality/type at will, providing you have the materials necessary. Runes can be crafted before even reaching Skyhold or meeting Dagna, but you must complete either "In Hushed Whispers" or "Champions of the Just" as this is required to spawn the first merchant (Havel) that sells blank rune stones.
Arcane Horror Heart Effect
Used to craft: Master Cleansing Rune