Thassarian and Koltira, inspired by that panel in the Death Knight manga.
They used to do this a lot before the whole Ebon Blade business. They got bored and restless when there weren't enough killing to do.
Koltira enjoyed jumping Thassarian under various circumstances, that guy did kill him after all.
Thassarian thought Koltira wouldn't really kill him, now they were brothers in undeath... he wasn't completely sure, though.
I wonder if there will be any progress of their story by the end of MoP. I don't even have MoP. BUT I WOULD. If enough story finally happens.