1. The problem is to rewrite the expression $\sqrt{-66}$ as a complex number and simplify all radicals. 2. Recall that the imaginary unit $i$ is defined as $i = \sqrt{-1}$. 3. Using this, we can rewrite $\sqrt{-66}$ as $\sqrt{66 \times -1} = \sqrt{66} \times \sqrt{-1} = \sqrt{66} \times i$. 4. Since $\sqrt{66}$ cannot be simplified further (66 factors as $2 \times 3 \times 11$, none of which are perfect squares), the expression in simplest form is $i \sqrt{66}$. 5. Therefore, the complex number form of $\sqrt{-66}$ is: $$\boxed{i \sqrt{66}}$$