Susurrus in umbris

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In the one exchange they do have, Gortash makes a point to threaten Isobel and mock Ketheric's attachment to her, but think how much of that mocking bravado must have been fueled by pure jealousy and bitterness.

Ketheric Thorn cast aside his goddess (twice!) and is ready to doom the entire world just to be able to resurrect Isobel. His participation in the Absolute plan is only to pay Myrkul back for Isobel's life. And even when she forsakes him, he still considers all the atrocities he committed, and selling his soul to Myrkul, a fair bargain.

To see that level of devotion, and know that your own parents couldn't even be bothered to keep you around, and instead choose to sell you off... Man, that must sting. I can easily see Gortash gnawing his teeth every time the subject of Isobel comes up, covering up his envy with empty mockery.