While you are back with Beniel, he mentions his explorers have discovered where General Terus is hiding. That's your next mission: find Terus and kill him. That would be too much for the legion and the undead would wander without anyone who organizes them.
As arriving there will be difficult, you have to kill a number of undead soldiers before finishing with this. You have to kill at least 20 more undead soldiers before the final attack. You begin in the forests again, and you kill the undead soldiers nearby. Once they are weakened enough, head north-east to the campsite where Terus is hidden.
At the end of the camp you find Terus, The General Of The Undead Legion. He is escorted by a couple of undead centurions, and a couple more of sorcerers: it's gonna be a hard battle. But that's why you are here!
After a great fight Terus and his escort are dead, and the Undead Legion will be a history from now... at least for the next 100 years!