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Selune and Shar have been rivalling since the beginning of time. Shar orders Viconia to "steal" this Selune child and groom her to become a Shar justiciar. How can Selune let this happen without blasting back at her twin sister ?

Also when did this happen ? Even though half-elfs have longer life spans than humans, I don't think SH is more than a century old when the story begins. So the kidnapping probably happened after Ketheric's defeat by the Harpers and his tbetrayal of Shar to turn to Myrkul. Also by this time, Dame Ailin was already imprisoned in the Shar Gauntlet, which had been desecrated and abandoned by Ketheric.

Did Shar (as a Goddess) have some foresight, knowing Shadowheart was predestined to restore the temple at Moonrise towers and kill the Selune Aasimar who was now being used in the service of Myrkul ? Was that why Shar chose SH as the object of Viconia's "mission" ?
Did Selune ( as a Goddess) have some foresight, knowing Shadowheart would preserve a spark of her Selunite upbringing, and would ultimately betray Shar, release Dame Ailyn and lift the Shadowcurse ? And therefore She (Selune) allowed this to happen (for the greater good) ?

So many stories are possible.