Egg of Hakkar

The Egg of Hakkar is used to summon Avatar of Hakkar in the Sanctum of the Fallen God in the Sunken Temple for the quest. It will not be removed from the player's inventory once the quest has been completed and can be used to summon Avatar of Hakkar in further runs of the Sunken Temple. There is no way to re-acquire the egg after completion of the quest if it has been destroyed by the player.

Source
This item is provided for the quest.

Before Patch 4.0.3a
The Egg of Hakkar used to be provided for the quest until patch 4.0.3a. It was turned into the during that quest as part of the  quest line. Players wanting to summon Avatar of Hakkar after completion of the quest had to use Yeh'kinya's Scroll.