「The Last of Us Part II」は感染症によってゾンビのような怪物になり果てた人間がはびこる、荒廃した世界を舞台としている。 From Jisho the word means completely, so does it mean in this passage that the people become completely zombies?
There are more than two handfuls of V果てる compound verbs, though I'm hesitant to say that it is fully productive.
It adds a nuance "thoroughly and irreversibly" to the base verb (呆れ果てる, 消え果てる), and often carries a connotation of "ruined" (荒れ果てる, 朽ち果てる). Few instances actually translate into "wholly" in number (絶え果てる).
なり果てる is なる "turn into" + 果てる, which suggests that their humanity (or that like) has been perished. Incidentally we occasionally use 変わり果てる to describe a body of violent death.