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I thought of that, but I don't think it works. Two reasons:
A - It's mentioned that the guards that are there are extremely powerful. Specifically, it's mentioned that the guys standing outside the warehouse are "not rent-a cop security", and that they are "the best money could buy" (around 12 minutes), and that the guys inside the warehouse "have received advanced training" and have "tasted deep magic" (around 26 minutes).
B- I can't think of a way to rationalize the heirarch using the police to get rid of magical opponents whose abilities he could forsee (like the father), but going alone to face unknown threats. Did he become the most powerful osteomancer in the west by being careless?
A - I get the impression that the invisibility glasses that Daniel wears were one of a kind, from a kind of magic that the Hierarch had not known about. The Hierarch did specifically make a comment about how it was very clever that he used two seperate kinds of magic for invisibility. It's highly possible that John Blackland, before he and the rest of the cabal were killed, was planning to use those to either filtrate the Hierarchy or help someone else infiltrate the Hierarchy, most probably to assassinate the Hierarch.
B - The name given to the assassination of the osteomancer's cabal was called the Night of the Long Knives. In the real world, the Night of the Long Knives of 1934 was a political power grab against the Nazi SA and other dissenters by the Nazi SS, who answered only to Hitler. Thousands of people were arrested, assassinated, imprisoned, and/or tortured into false confessions of treason, and then summarily executed. While it is possible that the Hierarch *could* have taken out most of the Osteomancers individually by himself, he could not have personally killed more than a few before the rest of the Osteomancer's Cabal were alerted and took measures to escape or protect themselves. It had to be a fast decisive coup to simultaneously arrest/kill as many of Hierarchy's political enemies as possible before they could react. It is also important to realize that it's been a span of 6~10 years (since Daniel was still a young boy when his father died) since that night and when Daniel finally confronts the Hierarch. It's highly possible that in the intervening years, while the Hierarch has been consolidating power and smoking every magical bone he could find, that he has let his guard down from megalomania or just gone out-of-his-gourd from long term exposure to osteomantic magic.
Personally, my take is that John Blackland was *not* a well known Osteomancer before his death, although we can guess that he was a relatively important figure in the Cabal. From Daniel's description it seemed like his father was a black sheep, and perhaps John had taken steps to hide the extent of his powers from the Hierarchy. It may very well have been that John Blackland was *the* leader of the Cabal, and forced the police to kill him so as to not reveal the other members, and possibly to save his son from becoming a high-value target of the Hierarchy. As for the final showdown, put yourself in the Hierarch's shoes. You have a thief who's gotten past most of your best defenses, wearing very powerful magic that he obviously did not craft himself. Do you 1) kill him immediately and never find out who sent this stooge, until maybe next time when it might be a handful of powerful osteomancers here to assassinate you? or do you 2) capture him alive and see if he gives up some information before you drink his incinerated remains with tea? While it could have been written a bit better, I thought it was fairly that the Hierarchy was attempting to subdue Daniel alive, and was taken utterly by surprise by Daniel's counter-attack.