Hannay becomes Vlaew at the end of Exmortis 3.
At the end, Hannay is teleported back to the Ancient times. Let's recap his physical and mental status: he not only has lost everything, but even after he set things right by killing Vlaew he's denied the chance to even see his efforts bear fruit. Also, he has had his fair share of experience with betrayal (and by the end of Exmortis 3 even practiced it himself in the Evil route) and knows the Ancients and what they're capable of.Azrael being still alive in that time would mean he wouldn't lose his powers even if he killed him in the original timeline.It is quite plausible that Hannay, arriving in the time where Earth was still ruled by the Ancient, used his knowledge of Ancients and stolen magic powers to seize control of a part of Earth and lead to the war between the three ruling Ancients, defeating Kfafta, betraying Azrael and taking over the world, to be later sealed away by mankind and wait until his chance to create a Hand and make his comeback possible. Hannay is Kfafta the third brother reincarnated.
Hence his powers and how he can kill Azrael despite it being stated only an immortal can kill an immortal. Vlaew killing him just awakened his powers.