It's not a very important trick to learn, because it's mostly only relevant in goofy ass maps, but the trick is to select a bunch of units and then hotkey bookmark them. You know, ctrl-1 for example. Then select one unit, press Death and Decay, but before clicking where you want to cast, press 1 and it reselects the whole group. The "cast" is still pending so the next place you click, all the death knights will cast there.