It is currently the 13th Anniversary for WoW until the end of the month, and it brings with it new quests and new rewards:
WoW's 13th Anniversary has a quest 
The Originals, to defeat Blasted Lands Lord Kazzak, Azuregos, and the four Green Dragons for 50 and a chance at ilvl 900 loot which can titanforge to 915.
Boss Overview
Pick up the quest at Historian Llore in Stormwind Library or Historian Ju'pa in Orgrimmar, Valley of Spirits. The Originals is a daily quest that requires players to be level 60 or higher.
Description:
Long ago, great terrors walked our lands, and drove even the smallest to flights of heroism. I speak, of course, about Lord Kazzak, Azuregos, and the Dragons of Nightmare. Legends have it that so sought after were their treasures, that armies would camp overnight to hunt them down. We have felt disturbances in the timeways, coincidentally located where these giants once roamed. Would you mind investigating for us? Strange things happen during this time of year.
These bosses drop some loot highly prized by transmog collectors such as 
Strangely Glyphed Legplates and 
Gauntlets of the Shining Light to match the transmog set Imbued Plate (Recolor), and the weapons 
Staff of Rampant Growth, 
Nightmare Blade, 
Typhoon, 
Emerald Dragonfang, and 
Amberseal Keeper.
You can bonus roll with 
Seal of Broken Fate to increase your chances at getting this special loot!
* = unique appearance for transmog
Lord Kazzak
Lord Kazzak is a very popular world boss, returning over the years as Doom Lord Kazzak in Outland and Supreme Lord Kazzak in Tanaan Jungle.
His original version, Lord Kazzak, spawned in the Blasted Lands and could be kited to Stormwind to rain destruction on the city. He also dropped one of the quest items needed for the Priest weapon 
Benediction.
His abilities included 
Capture Soul which healed him on player death, 
Mark of Kazzak to drain player mana and then cause an explosion, and 
Twisted Reflection which heals Kazzak when a specific player is damaged.
Loot: 
Blacklight Bracer, 
Fel Infused Leggings, 
Ring of Entropy, Amberseal Keeper*, 
Doomhide Gauntlets, 
Flayed Doomguard Belt, 
Eskhandar's Pelt, 
Infernal Headcage, 
Empyrean Demolisher*, 
Blazefury Medallion
Azuregos
Azuregos was a very difficult and time-consuming boss due to the 
Mark of Frost ability which placed a 15 minute debuff on anyone slain, turning them into an ice cube. He also periodically wiped aggro, teleported to players, and reflected all spells back onto players.
Azuregos also had a sense of humor; he got fed up protecting the 
Blue Scepter Shard and gave it to the shark Maws. This started a lengthy questline to summon the boss, involving collecting pages from bosses, spending lots of gold on costly materials, and acquiring 
Recipe: Dirge's Kickin' Chimaerok Chops:
After his stint as a world boss, Azuregos became a questgiver in Azshara. With all the times players killed him, he wanted to stay in the spirit realm and fell in love with the spirit healer! Learn more about his role in the 
Farewell, Minnow questline.
In Legion, Azuregos has a more serious role in the Arcane Mage and Discipline Priest artifact questlines that take you to the Nexus.
Loot: 
Snowblind Shoes, 
Cold Snap, 
Fang of the Mystics*, 
Unmelting Ice Girdle*, Typhoon*, 
Crystal Adorned Crown, 
Drape of Benediction
Green Dragons
These dragons are best known as the Dragons of Nightmare in the Emerald Nightmare Legion Raid.
Originally, they spawned in Emerald Portals located in Feralas, Duskwood, Hinterlands, and Ashenvale, and had some shared abilities and loot.
All Dragons shared the 
Mark of Nature ability, which placed an aura on players that died during the encounter. When a player tried to engage the boss again, the 
Seeping Fog would put the player to sleep for 2 minutes instead of 4 seconds - severely limiting zerg runs.
Shared Loot: 
Gloves of Delusional Power, 
Dragonspur Wraps*, 
Ancient Corroded Leggings, 
Dragonbone Wristguards, 
Hammer of Bestial Fury*, 
Green Dragonskin Cloak, 
Acid Inscribed Greaves, 
Trance Stone*, Staff of Rampant Growth
Emeriss
Emeriss has the special abilities 
Volatile Infection causing AoE damage around a player, 
Corruption of the Earth triggered every 25% health lost which placed a DoT on players, and 
Putrid Mushroom which spawned a mushroom on corpses.
Loot: 
Dragonheart Necklace, 
Boots of the Endless Moor*, 
Circlet of Restless Dreams*, 
Ring of the Unliving, 
Polished Ironwood Crossbow*
Lethon has the special abilities 
Shadow Bolt Whirl which fired bolts of shadow damage and 
Draw Spirit triggered every 25% health lost which stunned players and summoned spirits which would heal Lethon if they reached him.
Loot: Gauntlets of the Shining Light*, 
Deviate Growth Cap*, 
Malignant Footguards, Black Bark Wristbands, 
Dark Heart Pants, 
Belt of the Dark Bog
Taerar has the special abilities 
Bellowing Roar which is a traditional dragon fear, 
Arcane Blast which deals damage and knockbacks, and 
Summon Shade of Taerar triggered every 25% health lost. At this time, Taerar will banish himself and summon three Shade of Taerar.
In Cataclysm, players defeat Taerar in Taerar's Fall.
Loot: 
Boots of Fright*, 
Unnatural Leather Spaulders*, 
Mendicant's Slippers, Nightmare Blade*, 
Mindtear Band
Ysondre has the special abilities 
Lightning Wave and 
Summon Demented Druid Spirit triggered every 25% heath lost which summons druids that Moonfire and silence.
In Cataclysm, players can aid Ysondre in defeating Taerar in Taerar's Fall.
Loot: Emerald Dragonfang, 
Jade Inlaid Vestments, Strangely Glyphed Legplates, 
Leggings of the Demented Mind*, 
Hibernation Crystal, 
Acid Inscribed Pauldrons*
