Found Urzok at the skywatch a couple days ago when "the enemy is moving against each other" happened. I had my level 30 Titan but there were so many enemies I couldn't finish Urzok off before he disappeared. Maybe take a team with you to do it, maybe I'm just awful at this game, who knows haha. Also there was no notification of a public event when I was there, he just showed up when the fallen and hive we're destroying each other.