Karya knew of the Mother of Masks.
She was the source of the concept of death gods in Runeterra, an exceptionally unique concept deity. Legend had it that she gave every death god a mask, allowing those who wore them to reap the souls of the dead and guide them to the realm of the deceased.
This ancient legend had been passed down since the dawn of Shurima. Later, Karya's title of "Desert Death God" was largely connected to his fondness for wearing masks.
But by then, Karya had already ascended. As an immortal, he would likely never touch the boundary between life and death, so he had treated such legends with little more than a smile. It wasn't that he disbelieved them; it was that believing or not believing made no difference.
He was immortal. Who would still bother studying death gods?
So he had only heard of the legend, without looking into it in detail.
It wasn't until he met Aatrox last time, and heard him recount his long journey of hardship, his understanding of life and death, and the many peculiarities of the Land of the Undead, that Karya gained a faint, subtle understanding of the veil between life and death.
That was why, at first, he hadn't realized why Maokai was acting this way. He had only sensed that something was a little off.
This great tree was clearly connected to life and death, and held the Mother of Masks in great respect. Yet when it spoke of Kindred and the Frog Spirit, it made no secret of its subtle disdain, as if it were closer to the essence of life and death than those proxy Death Gods.
Even when its life was threatened, and it was about to be buried and destroyed beneath the sand,