>>712660797
The side effects of each crest aren't listed out so unless explicit information is given then you kind of have to headcanon it. For example the Crest of Blaiddyd's effect is that it can double the power of a combat art at twice the durability cost. In universe that translates to Dimitri swinging his weapons so hard that they shatter on impact. Saint Cethleann is lauded as the greatest healer in the War of Heroes, therefore the Crest of Cethleann increases the effectiveness of healing magic. But some of them don't match their in game effect at all. Catherine theorizes that the Crest of Charon summons thunderstorms. But in game? It raises the might of combat arts...cool.

So where does sex crazed Marianne come from? Well when you fight Maurice during her paralogue he mentions how his blood is driving him insane and he reverts to a feral state. If you say that's a side effect of his Crest then the same or similar should apply to Marianne. Except instead of just making her hit harder in combat, she just because a crazy nympho based on my research on gelbooru.