Yeah the Kemmler thing
really surprises me. If the Warden can release the prisoners (as Alfred/Demonreach told Harry) why didn't he release some Ethniu-level power on the condition that it swore a magically-binding oath to destroy the White Council?
Maybe something to do with the 'starborn cycle' made Ethniu more freely able to act in the mortal world than would have been possible a century ago?
Still, even a half-dozen naagloshii assassins could have seriously messed up the White Council's war efforts, likely enough for Kemmler to win.
I assume that was either body hopping Capiocorpus style, or he'd performed a low-level Darkhallow that regenerated him slowly.
I thought it was what Corpsetaker tried to do in GS, except that Kemmler succeeded where she failed.
He definitely did do some kind of "low-level Darkhallow" however, since Mab says that eating spirits was the secret of the power that let him defy the Council. It's not just something he knew how to do but never got a chance to use - he actually gained power from it.