An excerpt from Chaldea Ace Creators' Interview:
Nasu: "I'll grant you the concept of death…" Well, as a Grand, he'd have to do at least this much! (laugh)
Shimazaki: At that time I thought, "Now we can win!". Everything so far was focused on this development. No matter if it was the CGs or the BGM, they were all too awesome! And it turns out that the reason he came to help was because of the bond made during the 6th singularity… it really felt like everything on this journey has been bound together tightly! But, there's something I'm curious about. The strike that granted Tiamat the concept of death, was it a result of his powers as the Grand Assassin after all?
Nasu: Personally, I feel that joining Chaldea means that you must throw away the right to being a Grand that exists for the sake of saving all mankind. Still, even after discarding that position, the strength of King Hassan's Saint Graph itself is still Grand-class. It doesn't mean that once he loses his Grand position and becomes a normal Assassin, his Saint Graph would immediately lose power. So, by using all the might in that superb Saint Graph, the strike that "granted the concept of death" could be performed. And afterwards, King Hassan's Saint Graph became that of a normal Servant's.
Essentially, King Hassan's power, up to the point where he imposed the concept of death on Tiamat, was the power he constantly wields as a Grand Servant. Anything after that is him as a regular Servant.
Romani:
The spiritual core expansion has stopped, and the magical reactor cores, are continuously being rebooted!
The injures sustained as it fell into the underworld is being healed!
It's appearing! It's the -
(Tiamat's art)
That's the true form of Beast II!
The imposition of the concept of death goes hand-in-hand with the strike itself, so the AP of King Hassan as a Grand scales to Tiamat's draconic form. He won't scale completely, since her Self-Sealing is still at work, but at the same time, she had begun regenerating to full health by the time she entered her true form - which is the one that King Hassan struck.