Here's a definition of 'serendipity' from the Oxford Dictionary: "faculty of making happy discoveries by accident". This week, I've made a few serendipitous discoveries, not least how well my view of how creativity involves talent, hard work and (you guessed it) serendipity chimes with that of Paul Hatcher, an artist who combines all three. You'll find his work at http://thedrawingsofhatch.blogspot.co.uk and http://thehouseofhatch.blogspot.co.uk.
Although serendipity is a great thing to find in real life, writers should beware of using it more than they can avoid in fiction. Along with its sibling, coincidence, it always smacks rather of pulling rabbits out of hats i.e. there's no logical reason for something to happen, it just does. Readers nearly always find it dissastisfying and unbelievable. Put it this way. When Queen Anne, wife of Richard III, died there was an eclipse. This is a historical fact, so it's a fascinating coincidence. If the queen in your fantasy novel breathes her last just as the moon hides the sun, it seems far-fetched. After all, what are the chances? I'd never say 'never', but use with care.