Why does this code give me a NameError when I run it?

I'm trying to code a text based adventure game in python, but I can't get the battle system to work. This is what I have so far:

import time
import random


battle_chance = 3

enemies = ['A hoard of Goblins', 'A skeleton army',  'A large troll']

random_index = randrange(len(enemies))
def init_battle():
    while True:
        x = randint(0,5)
        time.sleep(2)
        if x == battle_chance:
            print(enemies[random_index], "Has approached you.\nWhat do you do?")

I always get an error and I can't figure out how to fix it.