This wouldn't seem to be an issue at first glance. After all, Multiple Curses of the Elements from multiple Warlocks don't stack, right? Here's the deal though. The Ebon Plague counts as a third disease for the purpose of Death Knight abilities that count diseases to deal damage, such as Scourge Strike and Blood Strike. When only one Death Knight can have Ebon Plague up, the other Unholy Knights are left out in the cold and have a part of their damage stripped away.