Sci-Fi/ Fantasy/ Post Apocalyptic. Secondary? character has a growth on neck which becomes a second head... Read 1990s. Spoilers ahead.

[Megan]

Read in Mid to Late Nineties Sci-Fi/Fantasy/Post Apocalyptic Character, pretty sure it wasn’t the main character, has growth on neck which eventually becomes a second head - and then the main head dies, leaving the second head, which appeared to be some sort of second coming of Christ.

[Llynnya]

This is a long shot.... She has a growth that eventually covers her whole head and she is blinded because her eyes are covered for a large part of book until the growth "cracks open" via her new powers to heal earth.

Swan Song

In a wasteland born of rage and fear, populated by monstrous creatures and marauding armies, earth’s last survivors have been drawn into the final battle between good and evil, that will decide the fate of humanity: Sister, who discovers a strange and transformative glass artifact in the destroyed Manhattan streets . . . Joshua Hutchins, the pro wrestler who takes refuge from the nuclear fallout at a Nebraska gas station . . . And Swan, a young girl possessing special powers, who travels alongside Josh to a Missouri town where healing and recovery can begin with Swan’s gifts. But the ancient force behind earth’s devastation is scouring the walking wounded for recruits for its relentless army, beginning with Swan herself. . . .

[Genesistrine]

It's a long time since I've read it but maybe Dad's Nuke? I seem to remember something like that in the last section but can't find any mention online...

[Ky]

That character sounds a lot like Mrs. Grales from A Canticle for Leibowitz who has a second head which she calls Rachel.

[Lobstergirl, au gratin]

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[Kris]

Megan, I copied a few book details to the topic header. Feel free to edit it.