Luminous Body
Rapid expansion (Inflation). A phenomenon more simply called "Luminous Body".
An excited state only possessed by True Ancestor royalty. When a True Ancestor's body is completely destroyed by some means, this reaction is said to occur. It was the result of Arcueid, a "small yet massive being", losing the container called 'body' and consequently having the compressed magical energy released, causing the scale of her existence to expand. Arcueid's consciousness exists at the core of the light, but since the barrier called 'body' doesn't exist, it is a state where among the excess amount of magical energy "an image of herself as she imagines it" is projected and diffused.
If a True Ancestor still brimming with magical energy is imperfectly destroyed, it will reemerge through just the energy that had been stored within its body and destroy human settlements in the form of a calamity, as a revenge mechanism. Inflation does not occur with Arcueid having her power stolen by Roa and fatally wasting away, but the Arcueid in Ciel route who killed Roa, was troubled by the bug that was
her love for Shiki, and could
no longer control her bloodsucking impulse could cause this phenomenon. Or rather, ended up causing it.