It means 'Shadowhunters: Looking Better in Black Than the Widows of our Enemies Since 1234'.

This phrase is a catchy expression that implies Shadowhunters have been successful in battle since 1234. It suggests that their ability to defeat enemies looks better than that of the widows of their enemies (who have been mourning the death of the loved ones). It implies that the Shadowhunters have been successful in battle since 1234 and have managed to outdo the widows of their adversaries in that time.