Temple of Atal'Hakkar

Temple of Atal'Hakkar (also known as Sunken Temple and more rarely, Lost Temple[1]) 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 unwittingly aided the Atal'ai by ensuring their efforts to summon Hakkar went on undisturbed.

History
Once, the temple was invaded by a night elf Lelior and a dwarf Ciarin. They fought against Atal'Hakkar Priests.

On the official site

From Blizzard's Official World Dungeons 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.