The Temple of Atal'Hakkar (also known as the Sunken Temple and more rarely, the Lost Temple) is a shrine erected by the Atal'ai trolls, led by their master Jammal'an the Prophet to the nefarious Blood God - Hakkar the Soulflayer. Believing that this was the intended site for Hakkar's reentry into Azeroth, the great dragon Aspect Ysera and her Green Dragonflight sunk the temple into the depths of the Swamp of Sorrows...but the dragons did not realize that it was the wrong location until it was too late.
Many of the bosses in the instance have some kind of prerequisite in order to encounter them. Atal'alarion will appear after activation of statues, Jammal'an the Prophet can only be reached by killing the 6 troll mini-bosses, Avatar of Hakkar must be summonned and the Shade of Eranikus will only be killable after the death of the Prophet.
The Temple may be entered by level 35 and the Meeting Stone is operational from level 45-??.
From World Dungeons on the old WoW site:
- Over a thousand years ago, the powerful Gurubashi Empire was torn apart by a massive civil war. An influential group of troll priests, known as the Atal'ai, attempted to bring back an ancient blood god named Hakkar the Soulflayer. Though the priests were defeated and ultimately exiled, the great troll empire buckled in upon itself. The exiled priests fled far to the north, into the Swamp of Sorrows. There they erected a great temple to Hakkar - where they could prepare for his arrival into the physical world. The great dragon Aspect, Ysera, learned of the Atal'ai's plans and smashed the temple beneath the marshes. To this day, the temple's drowned ruins are guarded by the green dragons who prevent anyone from getting in or out. However, it is believed that some of the fanatical Atal'ai may have survived Ysera's wrath - and recommitted themselves to the dark service of Hakkar.
From Atal'ai Exile:
From Brohann Caskbelly:
Swamp of Sorrows
-  (Quest is in Swamp of Sorrows outside the temple in the water)
-  (Quest is in Tanaris):
-  (Quest is in Ratchet)
Temple of Atal'Hakkar
-  Class Quest (Dungeon)
- Sunken Temple
-  Into the Temple of Atal'Hakkar (Dungeon)
-  Into the Depths (Complete)
-  Secret of the Circle (Dungeon)
-  Jammal'an the Prophet (Dungeon)
-  The God Hakkar (Dungeon)
- Un'Goro Crater
-  Haze of Evil (Dungeon)
All classes will have a class-specific quest chain that will lead them into the Temple of Atal'Hakkar. Each quest chain ends with a choice of 3 rare items. For an analysis of the item choices, see: Sunken Temple Rewards (work still in progress).
-  (Starts with Shaman trainer)
-  (Quest is in the Western Plaguelands)
- [??] (Paladin trainer in Ironforge or Stormwind)
-  (Champion Bachi or Champion Cyssa Dawnrose)
-  (Requires killing stuff in Western Plaguelands)
Note: You get this quest (the first one) from Mage trainers in any major city
- A Troubled Spirit (quest is in Swamp of Sorrows)
- An Imp's Request (You get this quest from Warlock trainers in any major city)
A good group to have for this instance would consist of a tank, a healer, preferably a Mage for AoE, and two damage dealers. AoE's are very helpful, as there are many packs with non-elites. The instance offers a lot of opportunity for crowd control, and magic dispels make it a lot easier, as there a various negative effects (Fear and Sleep being the most dangerous). One of the two damage dealers should be able to tank, or have a pet that can tank, for the Shade of Eranikus fight.
Each of the classes bring something useful to this instance, and as much depends on player skill and character levels as does group composition. That said, it's very nice to have reliable Crowd Control from a Mage, good healing from a suitably geared and/or spec'd Paladin, Priest, Druid or possibly Shaman, as well as some form of Out of Combat Resurrection and Wipe Recovery from a Paladin, Priest or Shaman unless the group is overpowered for the instance, very competent or just plain ready to do a corpse run or two.
"Phat Loot" NPCs
Update for Patch v2.3.
- End Boss: Avatar of Hakkar 50+ and Shade of Eranikus 50+
- Mini Bosses: Jammal'an the Prophet, Atal'alarion, Dreamscythe, Weaver, Morphaz, Hazzas and Ogom the Wretched.