This is a theory a friend and I came up while playing as Githyanki and finding out the following.
We know from Shadowheart's backstory, she undertook a suicide mission with a group of Shar's worshipers.
From BG3's official page:
A loyal cleric of Shar, Shadowheart is the sole survivor of a holy mission undertaken on the Mistress of the Night’s behest. She alone must deliver a relic of immense power to her coven in Baldur’s Gate, while threatened by a strange new magic that is burgeoning from within.
Shadowheart has in her possesion a mysterious at the start of the game, if you examine it you can see that it is engraved with Githyanki scripture. After succeeding on a DC10 investigation che