Death

The Sage's words reminded him of his grandmother's prophecy that his father had passed on to him.

He wasn't supposed to die at the hands of a Martial Sage.

It was possible that simply hearing that prophecy changed the future in a way that caused the prophecy to be true.

Rui didn't know.

And he didn't care.

He had no intention of just accepting his death passively.

If he was going to die, he would much rather die fighting and die adaptively evolving to his very limit. 

His plan was simple.

All metabody techniques at once, including the Megamind technique.

Simultaneously, use the Yin-Yang Spear to supercharge the Niflheim Domain like he did with the Muspelheim technique against Master Uma.

Together, he may very well be able to obtain the power of a Martial Sage for a single moment.

And then he would cease to exist.

One of the forbidden techniques was absolutely deadly on its own.