This achievement is gained upon defeating the following rare monsters in the Heart of Fear:

The Threshold: Alsecht the Believer
The Threshold: Deathfist
The Threshold: Harbinger Krask
The Threshold: Ocululor
The Threshold: Ulzschanoth
The Threshold: Ythan the Gutripper
The Rebirth: A Hoary Gargoyle
The Rebirth: Enasni the Demented
The Rebirth: Glubbus the Fleshmelter
The Rebirth: Ixmilsh the Terrortangler
The Rebirth: Silandra the Cruel
The Rebirth: Torflog the Impaler
The Rebirth: Vizlix the Deceiver
The Rebirth: Zixial the Scaremonger
The Epicenter: A Herald of Fear
The Epicenter: A Vision of Fear
The Epicenter: Grizelna the Mad
The Epicenter: Simira the Dreadwidow
The Epicenter: Soulhollow
The Epicenter: Yilsme the Harbinger of Death