The Assassin and the Empire

Arobynn arranges for her to be imprisoned at Endovier rather than executed, which is in keeping with his character: he destroys her but keeps her alive, preserving the possibility of reclaiming her later. It also ensures she suffers the maximum available humiliation and pain. The salt mines of Endovier are the worst prison in Adarlan, reserved for the condemned rather than the merely punished.

Not strictly, but Throne of Glass carries considerably more weight when you know what Celaena has lost. Her grief, her guarded nature, and her complicated feelings about Arobynn in the main series are all rooted in the events of this novella. Many readers who skipped the novellas initially report finding them essential on re-read.

Farran's role is confined to the novellas; he is not a major figure in the main Throne of Glass books. His significance is as the instrument of Sam's death, not as an ongoing antagonist. The villain who carries the weight of Arobynn's betrayal into the main series is Arobynn himself.