After a great war in Averia, a former female assassin seeking redemption is taken under the wing by a noble family, where she must learn to navigate high society while keeping her real identity a secret from the people that hunt her.

INTENTION: a new life. (But why? Why a new life? What’s she running away?)

OBSTACLE: old foes. (Old foes? Old agents? I feel this needs work too. How can we make it really bad for her?)

A female assassin sets out to start again and marries into a noble family, but has trouble keeping her past life a secret when her old foes set out to find her.

Yqwertz is right. We need a stronger inciting incident. I think we need to be more specific with everything. Why start again? Why marry into a noble family? Who is after her?

Here’s another quick shot at it…

After the love of her life is killed during a mission, an assassin sets out to start again and falls in love with a new man – only for her old foes to track her down.

Second one is probably better. Could replace “sets out to start again” with something else. Something quicker.

My examples are still quite muddled, but hopefully they’ll help you get the ball rolling.

Good luck!