Yes, looks like Averios.6012 is correct; the problem is Tonn's combat status when the last cutscene starts. Before initially starting Tonn, I ran to the third tower and cleared all around it, including veteran and the kraits that come out of the sea. Then I ran back to entrance, started it, ran to tower 1, then to tower 2, then to tower 3 without killing anything else. Tonn blew up third tower while not being in combat, the remaining risen spawned and got killed, and afterwards I could talk to Tonn to finish.
