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I was thinking about it !! Caleb has very ambiguous feelings about that : on one hand he’s happy to have this unique chance of growing old with his lover, but on the other he feels guilty of making him change his appearance all the time for his sake.
On his deathbed, he asks Essek to take the illusion down, and so he does. Caleb cries as he strokes the perfect youthful face, kisses the plump lips, and tells a heartbroken sobbing Essek he loves him.
May I present to you:
Essek casting Seeming as Caleb grows older so he and Caleb can be old together
Also for your consideration:
Essek taking Frumpkin as his own familiar since he and Frumpkin are going to live a long time without Caleb, and will out-live all of the Mighty Nein