Markdown textfield

config.yml

---
Name: markdown-title
---
SilverStripe\Core\Injector\Injector:
  Markdown:
    class: App\ORM\Markdown

SilverStripe\CMS\Model\SiteTree:
  db:
    Title: 'Markdown'
    MenuTitle: 'Markdown'

Markdown.php

<?php

namespace App\ORM;

use Exception;
use Psr\Container\NotFoundExceptionInterface;
use SilverStripe\Core\Injector\Injector;
use SilverStripe\ORM\FieldType\DBVarchar;

class Markdown extends DBVarchar
{
    private static $casting = array(
        'ToHTML' => 'HTMLText'
    );

    private static $escape_type = 'xml';

    /**
     * Render markdown as HTML using Parsedown
     *
     * @return string Markdown rendered as HTML
     */
    public function ToHTML()
    {
        try {
            $parsedown = Injector::inst()->get(\Parsedown::class, true);
            // remove the enclosing <p> </p> tags
            return substr($parsedown->text($this->value), 3, -4);
        } catch (NotFoundExceptionInterface $e) {
        } catch (Exception $e) {
        }
        return '';
    }

    public function Plain()
    {
        return strip_tags($this->ToHTML());
    }

    public function forTemplate()
    {
        return $this->ToHTML();
    }
}

You'd also have to install: https://packagist.org/packages/erusev/parsedown

To improve performance, use partial caching or caching within the class… whatever floats your boat.