All players will benefit from the Pandemonium buff during Season 19. For each stack of this buff, you’ll receive a small boost to movement speed and bonus damage, ultimately capping out at 50% movement speed and 100% damage at 1000 stacks. In addition, reaching certain killstreak goals will trigger a special effect and cause even more mayhem on the battlefield:

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: ?? ??