All 3 Uses of

inexorable

in

Frankenstein - 1831 version

But the old man decidedly refused, thinking himself bound in honour to my friend, who, when he found the father inexorable, quitted his country, nor returned until he heard that his former mistress was married according to her inclinations.†
p. 21.3 *

Frankenstein, your son, your kinsman, your early, much-loved friend; he who would spend each vital drop of blood for your sakes, who has no thought nor sense of joy except as it is mirrored also in your dear countenances, who would fill the air with blessings and spend his life in serving you—he bids you weep, to shed countless tears; happy beyond his hopes, if thus inexorable fate be satisfied, and if the destruction pause before the peace of the grave have succeeded to your sad torments!†
p. 90.8

Definition:

unstoppable or relentless