EventController.php

  /**
   * Init ICalendar Action
	 * Set Format to ICS
	 */
	public function initializeICalendarAction() {
		$this->request->setFormat('ics');
	}
	
	/**
	 * action iCalendar
	 * 
	 * @param \Codearts\Cevents\Domain\Model\Event $event
	 * @return void
	 */
	public function iCalendarAction(\Codearts\Cevents\Domain\Model\Event $event) {
		$this->view->assign('event', $event);
	}

ext_localconf.php

\TYPO3\CMS\Extbase\Utility\ExtensionUtility::configurePlugin(
  	'Codearts.' . $_EXTKEY,
		'ICal',
		array(
				'Event' => 'iCalendar',
		),
		// non-cacheable actions
		array(
				'Event' => 'iCalendar',
		)
);

ext_tables.php

\TYPO3\CMS\Extbase\Utility\ExtensionUtility::registerPlugin(
  	$_EXTKEY,
		'ICal',
		'CEvents'
);

ICalendar.ics

{namespace v=Tx_Vhs_ViewHelpers}BEGIN:VCALENDAR
<v:format.trim>
VERSION:2.0
PRODID:-//Prisma//Event//EN
<f:render partial="Event/Item" arguments="{event:event}"/>
</v:format.trim>

Item.ics

{namespace v=Tx_Vhs_ViewHelpers}
BEGIN:VEVENT
SUMMARY:{event.title}
DESCRIPTION:<v:format.plaintext><f:format.stripTags>{event.preReport.description}</f:format.stripTags></v:format.plaintext>
DTSTAMP;TZID=Europe/Berlin:<f:format.date format="Ymd">{event.dtStart}</f:format.date>T<f:format.date format="His">{event.dtStart}</f:format.date>Z
<f:if condition="{event.dtEnd}">
DTSTART;TZID=Europe/Berlin:<f:format.date format="Ymd">{event.dtStart}</f:format.date>T<f:format.date format="His">{event.dtStart}</f:format.date>Z
DTEND;TZID=Europe/Berlin:<f:format.date format="Ymd">{event.dtEnd}</f:format.date>T<f:format.date format="His">{event.dtEnd}</f:format.date>Z
</f:if>
<f:if condition="{event.externalLocation}">
LOCATION:{event.externalLocation}
</f:if>
<f:if condition="{event.internalLocation}">
LOCATION:{event.internalLocation.title}
</f:if>
END:VEVENT
END:VCALENDAR

setup.txt

plugin.tx_cevents {
	settings {
		iCalendarViewPid = {$plugin.tx_cevents.settings.iCalendarViewPid}
	}
[globalVar = TSFE:id = {$plugin.tx_cevents.settings.iCalendarViewPid}]
  page >
	page = PAGE
	
	page.config {
		no_cache = 1
		disableAllHeaderCode = 1
		xhtml_cleaning = none
		admPanel = 0
		disablePrefixComment = 1
		metaCharset = utf-8
		additionalHeaders = Content-Type:text/calendar;charset=utf-8
	}
	# render iCalendar for event
	lib.icalendar = USER
	lib.icalendar { 
	        userFunc = TYPO3\CMS\Extbase\Core\Bootstrap->run
	        pluginName = ICal
	        extensionName = Cevents
	        controller = Event
	        vendorName = Codearts
	        action = iCalendar
	        settings =< plugin.tx_cevents.settings
	        persistence =< plugin.tx_cevents.persistence
	        view =< plugin.tx_cevents.view
	}
	
	page.5 = COA
	page.5 { 
	  10 =< lib.icalendar
	}
[global]