Dahut tell to Carsa? -
(i) that she loves him;
(ii) that she was a virgin before him;
(iii) that she no longer wants to consummate her marriage to Gratillomius;
(iv) that Tambilis has told her that Gratillonius does want to consummate it -
(v) - and that his will to resist is crumbling;
(vi) that she will not mind if Carsa as King tries to lead Ys to Christ.
Closer analysis of her dialogue might reveal more. This is about as distorted, deranged and deceitful as you can imagine. (iv) and (v) grossly misrepresent Gratillonius but these lies are necessary in order to induce Carsa to challenge Gratillonius. Carsa thinks that he is protecting Dahut from something that Gratillonius wants and that she does not whereas the truth is the reverse.
What Dahut needs, for her own protection as well as for everyone else's, is a long period of exile and isolation with no one in speaking distance whom she can seduce or mislead.