We covered a lot of the announced mid-season updates, including the changes to mages, the AP item changes and the new Elemental Dragons in our mid-season update post. If you want more details, head over there.
You can also read the full patch notes here, but here's the important stuff:
1. Mage reworks!
Lots of them! Vladimir, Malzahar, Cassiopeia, Zyra, Brand and Vel'Koz received major updates, while Anivia, Annie, Fiddlesticks, Kennen, Swain, Syndra, Veigar, Viktor, Xerath and Ziggs received minor ones.
The bigger ones, on first glance: with spell vamp items removed from the game, Vlad's kit has extra healing added to it (on both his Q and his ultimate), while his E now charges up, doing more damage and costing more health the longer it charges.
Malzahar's new passive allows him to be immune to crowd control after avoiding direct damage for a certain amount of time, and his new W summons voidlings (which can summon other voidlings by attacking champions or large monsters or assisting in killing a unit). The zone that used to be on his W is on his ult now, making his ult do AOE damage (while still only suppressing one champion).
Cassiopeia has a new passive that grants her movement speed per level, which also means she can't buy boots. Her new W grounds enemies, making them unable to use movement abilities. Team Liquid head coach Locodoco has predicted this will make her the most viable in competitive play out of any of the reworked mages.
Brand's new passive increases his already high effectiveness in bunched team fights, as he does AOE damage to champions he hits with three abilities.
Veigar's passive has changed as well: he gains 1 ability power for each team he hits a champion with an ability. Coupled with his still-existing ability power stacking from killing units with his Q, Veigar will just keep stacking up ability power.
2. Objective changes
Dragon's completely different now! Instead of one Dragon respawning every six minutes, there are now four different Elemental Dragons, one of which (chosen at random) will spawn six minutes after the previous one has been killed. After one is killed, you will be able to see which one is coming next.
The Elder Dragon, a bigger and badder dragon, will spawn every 10 minutes starting at 35 minutes, which means there will be a maximum of three different Elemental Dragons in each game. If you happen to get two of the same kinds of Elemental Dragon in the same game, the effects will stack. For 120 seconds, Elder Dragon will give you a burn-over-time on spells and attacks, plus a 50% buff on all of your previously earned Elemental buffs.
Here's the permanent buff each Dragon gives your team!
Mountain Drake: Bonus damage to turrets and Epic monsters (objective control)
Infernal Drake: Bonus attack damage and ability power (fighting control)
Cloud Drake: Bonus movement speed (rotation control)
Ocean Drake: Bonus health and mana regeneration (siege control)
As for other objective changes...
Rift Herald now spawns at 6 minutes, and will not respawn. It's harder to kill, but the single-champion buff given lasts 20-minutes and does not go away if you die. It's a split-pushing buff, giving damage reduction and the opportunity for bonus damage when not nearby enemy allied champions.
Turrets deal more damage on repeated shots, and are now harder to kill.
Death timers have been reduced between 20-35 minutes, and boot enchantments have been removed.
3. AP Item changes
A lot going on here too, in conjunction with the mage reworks. Here are what appear to be the notable changes:
Chalice of Harmony/Athene's Unholy Grail: Now restores either health or mana, whichever is at a lower percent. Athene's also takes a percentage of damage you recently dealt to enemies and adds it to your healing and shielding of allies.
Catalyst of Aeons/Rod of Ages: New passsive: Damage taken is converted to mana, and mana spent is converted to health. Rod of Ages also costs less but gives less damage and mana.
Tear of the Goddess/Archangel's Staff/Manamune: Instead of granting mana regeneration, now refunds 25% of mana spent.
4. Other item changes
There are two new Hextech items! The GLP-800 fires AOE bolts that slow, while the Protobelt-01 has a short dash, which makes it seem pretty strong.
Devourer has been replaced as the attack speed jungle item by Bloodrazor, which gives 40% attack speed and gives bonus damage based on the target's maximum health.
Guinsoo's Rageblade is more expensive, and the AoE during Guinsoo's Rage has been replaced by Sated Devourer's additional on-hit effect.
Quicksilver Sash now only removes crowd control debuffs, instead of all debuffs (like Zed's ultimate, Trundle's ultimate, etc.).
The passive shields from Hexdrinker and Sterak's Gage no longer stack, ending what had been a popular hybrid offensive/defensive item build.
5. New splashes and upcoming skins!
Following a gothic horror theme, Iron Inquisitor Kayle, Cursed Revenant Nocturne and Black Scourge Singed will be arriving:
The new Lil' Slugger Trundle splash art is great:
League of Legends Patch 6.9 should be live either later today or later this week.