The Dreadblades tooltip is as follows:
"Strike at an enemy, dealing (95% of Attack power) Physical damage and empowering your weapons for 10 sec, causing your Sinister Strike,
Ghostly Strike
Ghostly Strike
Flagellation
Slaughter
Echoing Reprimand
Echoing Reprimand
Serrated Bone Spike
Serrated Bone Spike
Ambush, and Pistol Shot to fill your combo points, but your finishing moves consume 5% of your current health."
At the end of the beta, Slaughter was reworked and renamed Flagellation. Dreadblades hasn’t been updated to work with Flagellation it seems. When activating Flagellation after Dreadblades, no combo points are gained despite the ability saying any of the 4 covenant abilities would fill your combo points.