Yval becoming Yue's personal spirit wasn't something that came out of the blue. The pair had discussed it extensively the moment they met for the first time over a decade ago.
Yue was aware of the Herald's abilities, and she knew that despite Yval's power, it was nigh impossible for her to prevent the resurrection of the Titans. Yes, the Golden Dragon Progentior was powerful, but ultimately, she was just a ghost from the past.
Unlike the Herald, who could regenerate endlessly, Yval's soul would dissipate eventually. If they fought, perhaps Yval would have won the immediate battle… but the Herald would win the war.
Eventually, the Herald would gain control over the Necropolis, and the Titans would inevitably return.
Thus, to prevent that future from coming to fruition, Yue devised a plan.
What made the Titans immortal? Why didn't they die despite losing the war? Why did the Dragons choose to seal the remaining Titans and not kill them?
The answer was simple.
Their divinity.
