Luminous Body

Abrupt Expansion (Inflation). A phenomenon simply called “Luminous Body.”
An excited state that only the royalty of True Ancestors possesses. When a True Ancestor has their body completely destroyed by some sort of means, it activates as a reaction to that.
As a result of Arcueid, who was “something small yet heavy,” having lost the frame that was her body, the compressed mana was released, and the scale of her existence was expanded.
Arcueid’s consciousness is at the core of the light, but since the walls of her body are gone, she is in a state in which “the image of herself that she pictures” is projected · diffused at the mercy of the excessive amount of mana. A natural phenomenon created from her force of will so to speak.
A reprisal function through which, should “a True Ancestor that still has mana in excess” be unskillfully destroyed they will soon manifest with their accumulated energy alone, and destroy a human city by becoming a calamity.
With her power stolen by Roa and approaching her death by emaciation, Arcueid can’t make Inflation happen, but having killed Roa in Ciel’s route, caused a bug due to her feelings of love towards Shiki, and become unable to suppress her Vampiric Impulses, Arcueid could bring about this phenomenon. Or rather, ended up bringing it about.

Tsukihime Material I: Tsukihime Work Glossary