Weighted version of random choice with 1 of 1 guaranteed choices

So I have this bit of code that takes 5 weighted rarity levels and randomly selects one. These rarity levels all have 10 corresponding image files, when a rarity is selected, the code then randomly selects an image file of that rarity.

What I would like to do is over the course of X amount of rolls, guarantee 1 specific image file of the Epic rarity is always selected, but only once. I'm assuming this would need a check of some kind.

I'm sure this can be done, but I am just having a hard time wrapping my head around how to approach it. Any help is greatly appreciated.

# randomly selects a weighted level of rarity
def quality():
        A = 'Common'
        B = "Uncommon"
        C = "Rare"
        D = "Legendary"
        E = "Epic"

        loot = [ (A,50), (B,30), (C,14), (D,5), (E,1) ]

        choices = []
        for item, weight in loot:
            choices.extend( [item]*weight )

        roll = random.choice(choices)

        return roll
# randomly adds Aura from the selected rarity
def Aura():
    #print(random.choice(os.listdir(os.path.join('/PNG/Aura/', q ))))
    layQ = os.path.join('/PNG/Test/Aura/', quality() )
    qLen = len(fnmatch.filter(os.listdir(layQ), '*.png'))
    if qLen > 0:
        png = random.choice(os.listdir(layQ))
        roll = os.path.join('PNG/Test/Aura/', layQ, png)

        return roll