Atiesh, Greatstaff of the Guardian

Mage Atiesh with spell crit aura.

Atiesh was the 3rd and final Legendary item from Vanilla. In lore, the staff had been wielded by every Guardian of Tirisfal dating back 1000+ years. After Medivh was killed by Lothar, Khadgar, and Garona, Atiesh was taken to Dalaran for safe keeping. When Archimonde fucked up Dalaran the staff was shattered into 42 pieces (40 splinters, the head of the staff which Kel'Thuzad had, and the base which Brann Bronzebeard had, but lost in AQ because he's a fucking garbage dwarf).

In the shitty comics Med'an gathered the pieces of the staff and magically put it back together. However the entire story about Med'an has pretty much been retconned away because Blizzard decided that that storyline was dumb. What really happened was that players rebuilt the Atiesh in Vanilla times and when you went to Outland in TBC Khadgar fucking stole it from us. That mother fucking shit wizard is all smug about it to, always turning into ravens and shit around me.

Wielders of Atiesh in CSM

CSM's first 2 Atieshes as modeled by Kendoch and Zelf.

Completed during Vanilla

• Zelf: Awarded the first Atiesh for being the GM and good attendance
• Kendoch: Awarded the second Atiesh for being awesome and handling DKP, recruitment and cyber columbo responsibilities.

Completed during TBC

• Scynzy: Awarded the third Atiesh for near perfect attendance and doing big numbers. Collected about 1/3 of the necessary 40 splinters before the 2.0 TBC pre-patch released and we basically gave up on Naxx. Completed the remainder of his splinters in Atena's runs.
• Atena: Awarded to himself as payment for running Naxxramas Money Run. Collected the splinters after Scynzy finished his.
  

Source

To obtain Atiesh the player had to collect 40 Splinters of Atiesh which were drops from any boss within Naxxramas. Once the player had all 40 spinters they combined them to form the Frame of Atiesh. With the Frame complete the player then needed to obtain the Base of Atiesh from C'thun and the Head of Atiesh from Kel'Thuzad.

600 attack power!!

With all the pieces gathered the player then took them to Anachronos outside of the Caverns of Time to combine them. He would then tell the player that the staff would need to be exorcised of the spirit of Sargaras. To do this the player would gather 4 other friends and take the staff to Straltholme. Within was a patch of sanctified ground used to exorcise the staff. The fight with the demon Atiesh was pretty intense for 5 man content.
Optimal party included:

• Warrior tank provided Disarm one of the demon's Andonisus, Reaper of Souls
• Rogue to wield the Andonisus
• Shaman to provide Windfury for the Rogue
• More than one healer wasn't unusual either because of the sheer amount of damage during the encounter

So basically you just let your Rogue carry you to victory using the broken sword. After exorcising the demon, the player then took the staff back to Anachronos and he granted the player the rarest legendary in the game.
With the removal of Naxxramas in the 3.0 Wrath pre-patch, Atiesh was no longer able to be obtained. The new Naxx25 had no equivalent to the Atiesh.

Trivia

• The staff can open a portal to Karazhan. Much less useful these days as there is an actual portal to Kara in Dalaran.
• The staff has a special effect that occasionally attracts ravens overhead of the wielder. This effect was used as a minor glyph for shadow priests displaying shadow orbs.
• When you engage Shade of Aran in Karazhan with Atiesh in your inventory he yells "Where did you get that?! Did HE send you??".
• When you engage Echo of Medivh in Karazhan with Atiesh in your inventory or bank he yells "My Staff...a relic, yes, but MINE!".
• Frostbitten gathered 39 of the necessary 40 splinters in the TBC Naxx runs hosted by Met when Atena burned out after finishing his Atiesh before the 3.0 Wrath pre-patch removed Naxx and his chance at finishing the staff.
• Pablohoney was nominated for the 3rd staff... by himself.

Other Versions of Atiesh

Priest Atiesh with healing aura.

Warlock Atiesh with spell damage aura.

Druid Atiesh with mp5 aura.