http://na.leagueoflegends.com/en/news/game-updates/patch/patch-413-notes#patch-mages-and-assassins
Pretty big patch, I can't list everything here, so please check it out.
"Welcome to patch 4.13! With the World Championships on the very near horizon, we're ramping up our efforts to promote more diversity in competitive play (boy are you going to hear this a lot). While this patch might seem really big - and it is! - it's more about the breadth of changes than any deep shakeups. What that really means is we've got about 40+ patch entries, but each change alone isn't going to be very significant in isolation. So with strategic diversity as our target in 4.13 (and this will be the same for upcoming patches leading to worlds), we've gone ahead and divided up our sections based on specific groups or themes. Under each category you can see our overarching goals and philosophies as well as deeper context on each individual champion, item, or summoner spell. Remember you can navigate through it all via our table of contents on the top right of the screen! Included in patch 4.13 is also a gameplay update for Sona! While Sona hasn't been on the very strong or very weak ends of the support spectrum, we saw a cool opportunity to give her more meaningful (and recognizable) gameplay without heavily changing her aura-focused playstyle. We'll have more context below in her gameplay update, or you can check out the Sona gameplay update page! Regardless, on with the patch notes!"