https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles

Most of the aria that denotes an area is about describing the user interface elements, they're not really about describing the content.

You could use invisible text, for the screen reader only:

<span class="aria-label">THIS SECTION IS IMPORTANT</span>

It may just be easier to use a title, with the word IMPORTANT in it? Such that all users know it's important.