Q: Can the spiritual core of a Servant be thought of as a vital area that, if destroyed, would cause instantaneous death, like a human’s heart or brain?
A: The heart and brain are certainly vital organs. Though they are different from the spiritual core, they can be considered organs directly connected to the core.
When the spiritual core is weakened through mana consumption or damage to the physical body, powerful mana, spells, and Noble Phantasms can no longer be materialized, and the Servant will disappear.