To simplify the expression \((x-5)^3 - 2y(5-x)^2\), we can first expand it step by step.

1. We notice that \((5-x)^2\) can be rewritten in terms of \((x-5)\):

\[
(5 - x)^2 = (-(x - 5))^2 = (x - 5)^2
\]

So we can rewrite \( -2y(5-x)^2 \) as:

\[
-2y(5-x)^2 = -2y(x-5)^2
\]

Putting it all together, the expression becomes:

\[
(x - 5)^3 - 2y(x - 5)^2
\]

2. Now we can factor out \((x - 5)^2\) from the two terms:

\[
(x - 5)^2\left((x - 5) - 2y\right)
\]

Thus, the simplified expression is:

\[
(x - 5)^2\left((x - 5) - 2y\right)
\]

This is the final simplified form of the given expression.