Add prefix text to category title

add_filter('get_term_metadata', 'bch_change_category_title', 10, 4);
function bch_change_category_title( $null, $term_id, $key, $single ) {
    if ('headline' == $key) {
        $term = get_term( $term_id);
        return 'History of Unit '.$term->name;
    }
    return null;
}