How do I output HTML from a text_long field in Twig?
Normally you would configure the display of the field in UI.
You can do this in code by using the render element processed_text.
PHP
  $build = [
    '#type' =&gt; 'processed_text',
    '#text' =&gt; '&lt;p&gt;Hello world&lt;/p&gt;',
    '#format' =&gt; 'full_html',
  ];

TWIG
In twig you would use instead of the extremely unsafe {{ event.getExternalDescription | raw }} this code:
  {% set build = {
    '#type':   'processed_text',
    '#text':    event.getExternalDescription,
    '#format': 'full_html',
  } %}

  {{ build }}

The safety of the text format depends on the configuration of the text filters and the content of the field. Apply the same security considerations as you would for text formats in ckeditor.