At home as a youngster, Han and Leia's son acted out using his untrained Force powers causing massive random destruction, so they sent him away to train with his Uncle Luke to 'settle him down'.

Somewhere between there and the massacre of the Jedi School, something happened to turn Ben Organa-Solo to the Dark Side, right under the eye of its greatest champion. What was it, who was it, and how did Luke not see it before it was too late?