If a player receives a spell from a dark gift in Curse of Strahd and survives the saving throw to become evil…

if a player receives a spell from a dark gift in the Amber Temple in Curse of Strahd and survives the saving throw to become evil, what mental statistic would that player use for the spell save DC? Example: Dwarven Fighter accepts dark gift and gains Cone of Cold I’d go with Charisma

— Mike Mearls (@mikemearls) August 22, 2018