The flip side of that 'evil hollows out spirits' factor:
If the spirits in question are either walking nuclear reactors or the power that sets them in motion, hollowing them out is when they're at their very most dangerous. They're walking nuclear disaster zones the entire time but at peak strength they can and do weaponize it and properly control it it as horrifyingly powerful and deadly fallen angels.
When the decay factor sets in you have feral roving walking Chernobyls spewing soul-annihilating radiation and burning the world in fire. They're pale ghosts of themselves but that means you're looking at *living leaking nuclear reactors that leave entire landscapes as sterilized radiation-saturated hellscapes that produce all the unlovely effects of radiation poisoning*.
In short late!stage decayed Varda and Ilmare/Sinmara are both feral monsters and pitiable in their own way but if not for Nessa the personification of FTL late-sage Varda just has to exist to turn all of Arda into a nuclear hellscape. Varda goes from a smug and arrogant devil-figure with delusions of rhetorical grandeur to a feral snarling creature in simian postures given a rough treatment by Este to allow her to go into the Doors with something like sapience.
Ilmare/Sinmara goes from a very Sauron-like God-Empress mentality to straight up Surtr 'BURN IT ALL' mentalities and has a suitably Ragnarokian conflict.
Sinmara of Muspeldor is less an equivalent of Sauron, though, and straight up the whole time is a Norse mythology reference to Surtr and Muspelheim down to Mordor's equivalent going from "Black Land" to "Land of the World-Destroyers." The reason people go along with someone who calls herself "World-Destroyer" is "Obey me or you die by the hands of my handheld nuclear weapon" is a very, very convincing argument. Plus Cosmic Horror starlight makes people feral and prone to obey the Maiar of the Star-Kindler who can and do wield the harmonics to provide an order reliant on a powerful commanding presence but aside from them they're warlike monsters that go a-marauding and only survive into the later Ages because they die like rabbits and they breed like rabbits.