The Feat of Strength [Unholy Determination] for killing all of the red dragons in the Sanctum has been removed for the live version.
So if you just committed a massacre and are here looking for word on why you got no achievement, that is the reason. In the words of Blizzard:
"Feedback from the community noted that a missable achievement was unfun for the players that missed it. While we've chosen to remove the Feats of Strength granted in the questlines overall, the voice line from Bolvar will still play if players do choose to go down this path as Death Knights.
As was alluded to, these Feats were not meant to go live, as we had previously decided that they were misaligned with our general look towards achievements, namely that they were unobtainable if missed in a non-repeatable quest area. Again, these will no longer be obtainable in 7.2.5, and for players who did obtain the achievement it will be removed."
And for those that had it removed:
"I can appreciate that no-one likes having something taken away from them. To try and clarify a little bit why we came to this decision; during the course of the PTR testing feedback from the community overwhelmingly pointed out that a missable achievement was both unfun for players that missed it, and unfun for players that felt compelled to slaughter innocent red dragons to achieve it.
I do also want to mention that, while the Feat of Strength will be removed, the special voice line from Bolvar will still play if players choose to follow this particular route."