Kahn started the Synthesis divine ability while using the Cthulhu's core and body to merge with Blackwall instead of choosing it to power himself up to make up for the lost levels and become a 4th stage saint instead.
Why?
Because using your Soul Essence and sacrificing your levels also made his body unable to level up for a considerable amount of time.
While healing Kahn, Armin already told him that given his rank and levels, he'd need at least 7 days to make a complete recovery and become able to absorb world energy again.
So instead of waiting, he decided to evolve Blackwall who was the only candidate that fit the bill.
Besides, System had already informed Kahn about how Blackwall could gather different Primordial Titan bloodlines and inherit their powers altogether ever since their time in Verlassen. And now… it was time for the Kronos general who stayed the weakest in the current story arc so far, to gain an immense boost.
