Found this on Steam Community - hope it helps:
The Spiritualistic Fallen Empire owns 4 Holy Worlds. Defeat them in a battle and conquer those planets to get the Achievement
Holy Worlds are Gaia planets with 100% habitability, but they have a special icon in the upper left of their planet screen (the usual place where you see positives or negatives for a planet)
They also have special names:Pristine Jewel, Walled garden, Prophet's Retreat, and Emerald Mausoleum (I THINK those are the 4 names, i'm hazy on it still)
To get all 4 to spawn in one world, set your amount of habitable planets to max and use a large galaxy.