Bug in campaign 3, mission 3, when playing as the Necromancer.

After obtaining two out of three artifacts, you proceed to the last city of mages, where Cyrus and the Ring of Sinners are located. After defeating the last garrison, you move forward a bit, the cursor changes to an hourglass, and the game freezes, with only fast loading and exiting via Alt+F4 working. Please advise on what to do.