Replace Taxomony Description Field with Visual/WYSIWYG Editor

You can use a {$taxonomy}_edit_form_fields action hook to add html to the term edit table. In that HTML you can remove description textarea and add tinymce editor

add_action("{$taxonomy}_edit_form_fields", 'add_form_fields_example', 10, 2);

function add_form_fields_example($term, $taxonomy){
    ?>
    <tr valign="top">
        <th scope="row">Description</th>
        <td>
            <?php wp_editor(html_entity_decode($term->description), 'description', array('media_buttons' => false)); ?>
            <script>
                jQuery(window).ready(function(){
                    jQuery('label[for=description]').parent().parent().remove();
                });
            </script>
        </td>
    </tr>
    <?php
}