Apple is aware of an iMessage issue caused by a specific series of unicode characters and we will make a fix available in a software update.
Until the update is available, you can use these steps to re-open the Messages app:
1. Ask Siri to “read unread messages.”
2. Use Siri to reply to the malicious message. After you reply, you’ll be able to open Messages again.
3. In Messages, swipe left to delete the entire thread. Or tap and hold the malicious message, tap More, and delete the message from the thread.
Source: Apple Inc.
Apple promises fix for ‘Unicode of Death’ iMessage bug – May 27, 2015
Random string of Arabic characters can crash nearly any Mac, iPhone or iPad – September 3, 2013