In a recent update on the patch 5.3 PTR, Blizzard applied an update that applied some unexpected changes to the death knight unholy tree, most of them nerfs.
This has, unsurprisingly, left many death knights at a loss. After patch 5.2 seemed to cure unholy's woes and allow it to at least be a viable and fun alternative to frost, these nerfs came as a surprise. Frost is so far still the more damaging death knight DPS spec in patch 5.2, so the idea that unholy is the one that got nerfs was somewhat discouraging.
That said, it might not be as bad as you think. It's worth noting that in the long run, the Scourge Strike buff should cancel out the nerfs to Festering Strike and Unholy Might, at least in theory. This brings to mind one question as to what this shifting of damage toward Scourge Strike is actually meant to accomplish. Essentially, the main purpose here appears to be a nerf to festerblight.