First, the most dangerous and least desirable transformation. Pure warpstone gives a range in yards, refined warpstone in inches. Morrsleib with a clear sky can still trigger it while waning, while an overcast night is only dangerous when Morrsleib is full. A trip into the formerly Skaven territories confirms that ambient Dhar can trigger the transformation, but only at levels equivalent to recent Battle Magic use. You spend some time on equations trying to reconcile these numbers, and confirm that if one assumes Morrsleib is 'merely' a source of ambient Dhar, it should not be anywhere near sufficient for the transformation. But then, Morrsleib has never been merely anything, as the fact that its phase can sometimes be visibly different in places mere miles apart can attest.