Bland campaigns bring this expansion down.
Shadow of Death is actually set between Heroes II, and Heroes III. The campaigns focus on the rise of everyone's favorite Necromancer (Warlock in the original Heroes) Sandro. He has somehow regained his flesh, and decides that he wants to conquer the world. Four heroes who each have their own campaign are manipulated by Sandro. After they're manipulated by the man who is once again losing his flesh, they all meet and team up against him. Yes, the missions are as uninteresting as the story, unfortunately.
This expansion adds some new single player scenarios for people who can't get enough of those. There aren't any new features for the Random Map Generator in AB, but since Shadow of Death also contains the original Heroes III game I suppose that's to be expected. New World Computing wouldn't want to give SoD one of the prime features of Armageddon's Blade, after all.
All Shadow of Death really adds are some new campaigns and scenarios. This worked in Price of Loyalty for Heroes II, but the difference is that the PoL campaigns were actually memorable and fun to play.