The equation $x^5 - 2x^4 - 1 = 0$ has five complex roots $r_1, r_2, r_3, r_4, r_5$. Find the value of 
\[
\frac{1}{r_1^8} + \frac{1}{r_2^8} + \frac{1}{r_3^8} + \frac{1}{r_4^8} + \frac{1}{r_5^8}.
\]

Let's think step by step and output the final answer within \boxed{}.
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