The Absolute was the Unclaimed.
That's why the Absolute is at odds with Shar.
We see a similarity between Shadowheart and the Unclaimed for a storytelling reason in that regard. It showcases how Shar treats her followers and gives us some small insight into the Absolute's history. It also creates more drama for Shadowheart, who will be either fighting or joining this tragic figure turned monster.
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But how did the Unclaimed turn into the Absolute?
This is all guesswork, of course, but is it possible that the dead three claimed her? Myrkul, Bane, and Bhaal? And that's why we notice their symbols within all things Absolute.
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What does this have to do with Gith, Mind Flayers, and Cambions? I'm not sure yet.
