Standard disclaimers apply, as much as I wish they didn't and if CLAMP would like the story idea they are welcome to it :D
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Prologue
It knew eventually that the day would come. It didn't know when or how. Truthfully a being with a life span that encompassed hundreds and not tens of years, tends not to have an interest in the When or the How simply the knowledge that it Will. The meddling female had cast the spell, and human years are limited. It was simply a matter of waiting for the time when it will happen.
However this being knew that when the event the woman had set in motion came to pass. It would be left with an inferior subordinate, every way lacking in comparison to its current user. Why its servant had chosen such a timid child to be his possible successor was beyond its comprehension. Of course, the boy was talented in the spiritual arts, powerful and knowledgeable beyond his years, could even be described as gifted. He was also morally righteous and insufferably weak-willed, and also the furthest being in existence capable of providing for its needs. He lacked the edge to succeed the position.
And it knew that.
Therefore you can't really blame it upon the decision that this Sumeragi -whom its servant had become so severely distracted by- was insufficient.
And an inferior servant was simply unacceptable.
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Ok this is gonna be a short one, just a random tick I got in my head after a Christmas CLAMP binge.