This is a main storyline achievement and cannot be missed.
Stains of Heresy is the primary quest for most of Lostlight, the angels' outpost and the third realm you visit. You receive the quest when you return the Rod of Arafel to the Archon in the Crystal Spire.
The main portion of the quest has you completing the Ivory Citadel, a dungeon memorable for being where you unlock the Voidwalker (portal gun) and where you cleanse three aqueducts of a viscous ichor that kills you if you step into it.
After defeating the Scribe at the end of the dungeon, he reveals that the Archon has actually been corrupted the entire time (gasp!) and you must fast travel back to the Crystal Spire to fight him.
After defeating the Archon, you must simply fast travel back to the Tree of Life and take the newly opened portal to Shadow's Edge, the demon realm. The achievement pops as soon as you arrive.