The Sunken Temple of Qarn is not part of the main story and is a side quest given by the NPC Nedrick Ironheart in Vesper Bay (Western Thanalan). The quest is given at level 32 and the max level for this dungeon is 35. Any player above 35 will be level synced to 35 upon entry to the dungeon. The dungeon is located in Southern Thanalan.
The Sunken Temple of Qarn is perhaps the first really challenging dungeon players will encounter, with the first boss proving the most difficult. Be sure to avoid the Final Sting move from the bees by killing them quickly or stunning it, and run to a lit panel when Doom is cast on you. This will remove it.