Source: The Reaper's Right Hand, pg(s). 90
A radiant essence is normally eight feet in diameter, between 8 and 12 inches thick, and weighs approximately one and a half tons. The colour of each radiant essence depends on the type of celestial whose remnants form the radiant essence: yellow for angels, silver for archons, orange for agathions, and red for azatas. Most radiant essences are orange, due to being made from multiple types of celestials. Like most oozes, radiant essences are made of amorphous, corrosive material, which shines softly and is particularly harmful against evil foes due to the radiant essence's holy energy. Radiant essences can harden their membrane to avoid secreting this acid, but rarely does so. A radiant essence, once formed, never changes size.
When a celestial is destroyed, its quintessence is drawn to its home plane, but in some rare cases the most powerful celestials leave behind some residue that wants to continue fighting evil; several such residues can coalesce into a radiant essence. Some alchemists have tried to replicate their creation but always failed; some scholars attribute this failure to divine will. Radiant essences are sustained by their own biology, do not need to eat, and only secrete their acid to break down obstacles.
Radiant essences do not reproduce. When it dies, a radiant essence deteriorates, leaving behind globules which are particularly useful to anoint a holy relic or sacred site and evaporate after a few days, although good heroes rarely sacrifice radiant essences for this purpose and only make use of them when they discover a recently dead radiant essence.
Radiant essences are mindless and driven to destroy evil to the point of losing all self-preservation. The rare radiant essences that are not destroyed in battle can live for centuries, until they run out of holy energy. The death of an old radiant essence has never been observed.
Many religious scholars are fascinated by radiant essences and study them in order to better understand celestial destruction and rebirth; the radiant essences themselves are indifferent to this fascination. Sometimes, a good deity orders a devotee to transport a radiant essence somewhere else so their crusade against evil can be more useful; the radiant essence might be compliant or obstinate with its relocation. Some servants of good deities collect radiant essences from the Material Plane so their essence can be used to create powerful celestials.
Radiant essences rarely stray far from a holy site which they adopted as their home. Most are solitary, except in large buildings or celestial battlefields. Due to their single-mindedness, radiant essences sometimes pursue evil creatures far from their lairs, but lack the intelligence to retrace their steps; such radiant essences continue to wander and seek out evil foes until they find a different holy site and adopt it as their new home, much to the chagrin of the residents.
Celestials view radiant essences with both pity and sympathy as they contain the essence but not the personality of fallen heroes, and caution as radiant essences are mindless, unpredictable, and an example of what they might become if they fall.