Which will be another game changer thanks to the Pandemonium buff that will define Diablo III's Season 19. Where speed and damage buffs are applied during Kill Streaks with things like "Exploding Chickens" and "Frost Nova blasts" triggering every 15 Kills - going all the way up to 1,000. Also, Season 19 also introduces new Set Items - one for the Crusader and one for the Monk.

First off here's the description of the new Pandemonium Seasonal buff.

For the duration of the Season, all players will have a stacking buff that that persists as long as you have hit or killed a monster within the last 5 seconds

Each stack gives .005 movement speed and .001% bonus damage. This bonus caps at 50% movement and 100% damage (1000 stacks)

In addition, after reaching a certain number of kills in a row, a power is unleashed, dealing an amount of scaling damage based on player level and difficulty or Greater Rift level:

15 Kills: Exploding Chickens seek and destroy
30 Kills: A wide Frost Nova freezes enemies
50 Kills: Corpses rain from the sky
100 Kills: Five massive energy twisters are unleashed
150 Kills: Dark Geysers form beneath enemies
200 Kills: Treasure chests fall from the sky
300 Kills: A ring of fire engulfs everything
400 Kills: Meteors hail from above
500 Kills: Angels descend upon the battlefield to fight for your cause
1000 Kills: ???

Powers are tracked at a group level; this means only one power can be active in play at a time, regardless of the number of players in a group.

The Season will kick off proper this weekend with the Season 19 Journey offering up new rewards in the form of a Valor Portrait Frame and the Angelic Treasure Goblin pet.

2.6.7 also brings the first batch of new Set Items to Diablo III, starting with the Crusader an Monk. Both of which have seen some changes since being implemented on the PTR.

Here's the Crusader set and related Legendary Item.

New Crusader Set: Aegis of Valor

• 2-Piece Bonus: Attacking with Fist of the Heavens empowers you, allowing Heaven's Fury to deal 100% increased damage for 5 seconds. Stacks up to 3 times (multiplicative).
  
• 4-Piece Bonus: Hitting with Fist of the Heavens generates 5 Wrath and reduces damage taken by 1% for 5 seconds. Stacks up to 50 times
  6-Piece Bonus: Increase the damage of Fist of the Heavens and Heaven's Fury by 20,000%

Darklight

• Fist of the Heavens has a 100% chance to be cast twice.
  
• Additional Legendary Power: Increases the damage of Fist of the Heavens by [800-1000%].

And here's the new Monk Set and related Legendary Items.

New Monk Set: Patterns of Justice

• 2-Piece Bonus: Sweeping Wind gains the effect of every rune and movement speed is increased by 5% for each stack of Sweeping Wind.
  
• 4-Piece Bonus: Attacking with Tempest Rush reduces your damage taken by 50% and increases Spirit Regeneration by 50.
  
• 6-Piece Bonus: Hitting with Tempest Rush while Sweeping Wind is active will temporarily increase the size of Sweeping Wind after hitting over 30 times within a short period. Sweeping Wind damage is increased by 15,000%

Vengeful Wind

• Increases the maximum stack count of Sweeping Wind by 10
  
• Additional Legendary Power: Increases the damage of Sweeping Wind by [600-800%].

Won Khim Lau

• New Legendary Power: Hitting with Tempest Rush will activate Cyclone Strike, and both skills deal [500-600%] increased damage.

There are also a number of updates to Barbarian items, in relation to Whirlwind and Rend - for those details head here.