Formal:
* Ainsi, le lion tomba amoureux de l'agneau. (This is a literal translation of "so the lion fell in love with the lamb.")
* C'est ainsi que le lion tomba amoureux de l'agneau. (This emphasizes the sequence of events.)
Informal:
* Et voilà que le lion tomba amoureux de l'agneau. (This is a more colloquial way of saying "and so".)
* Alors, le lion est tombé amoureux de l'agneau. (This is a more casual and direct translation.)
You could also use a phrase that emphasizes the unexpected nature of the situation, such as:
* À la grande surprise de tous, le lion tomba amoureux de l'agneau. (This translates to "To everyone's surprise, the lion fell in love with the lamb.")
The best way to say it depends on the context and your intended meaning.