Catalyst

Kay and Ada achieved the impossible and defeated the StoneKing, but only they know about the terrible dangers beyond the Inter-World Alliance's borders - and within them.
All Kay wanted was to escape the shadow of his father, Lawrence Walker, the former council member responsible for dooming Ada's homeworld to destruction. But now Walker is back, his plans more ruthless than ever, and the world he left behind might be the final catalyst for a cross-world war.
Kay and Ada race against the clock to stop Walker and save a world from the deadly trap that cost Kay's mother her life. With the Alliance turning on itself and Ada's homeworld threatening war, Kay and Ada must overcome their most dangerous adversary yet before it tears the Multiverse apart.