How to add a custom vertical tab to node edit form?
If you're looking insert renderable elements into the form array in the same manner as the grouped vertical tabs, you can hook_form_alter() your way to this via adding a details form element:

function MYMODULE_form_alter(&amp;$form, $form_state, $form_id) {

  // @TODO: Add your own logic for filtering relevant node forms
  if (in_array($form_id, array('node_page_form', 'node_page_edit_form'))) {
    $form['example'] = array(
      '#type' =&gt; 'details',
      '#title' =&gt; t(''),
      '#description' =&gt; t(''),
      '#group' =&gt; 'advanced',
      '#open' =&gt; TRUE,
      'markup' =&gt; array(
        '#markup' =&gt; '&lt;p&gt;Random Markup&lt;/p&gt;',
      ),
      '#weight' =&gt; 1000,
    );
  }
}


If you're looking to add markup that's not relevant to your node forms, you might be better off subtheming Seven and adding your own node-edit-form.html.twig template to your subtheme with your own markup additions:

&lt;div class="layout-node-form clearfix"&gt;
  &lt;div class="layout-region layout-region-node-main"&gt;
    {{ form|without('advanced', 'actions') }}
  &lt;/div&gt;
  &lt;div class="layout-region layout-region-node-secondary"&gt;
    {{ form.advanced }}
    &lt;p&gt;Random Markup&lt;/p&gt;
  &lt;/div&gt;
  &lt;div class="layout-region layout-region-node-footer"&gt;
    {{ form.actions }}
  &lt;/div&gt;
&lt;/div&gt;