Cimmeria
The three raid instances in Cimmeria together comprise the Tier 1 raid content. Tier 1 Epic Armor, Epic Weapons and Simple Relics I can be obtained from these bosses.
Stygia
The raid instances in Stygia contain both Tier 2 and Tier 3.
Black Ring Citadel (Tier 2)
The Tier 2 raid dungeons are contained within the Black Ring Citadel (BRC). The building is comprised on three wings - Wing 1, Wing 2, Wing 3. Each of the bosses drops a Tier 2 Epic Armor, Weapons and Simple Relics II.
Thoth-Amons Stronghold (Tier 3)
Tier 3 involves fighting your way through TA's stronghold, eventually confronting Thoth-Amon himself. Tier 3 Epic Armor and Weapons, Black Dragon Armor, Void Weapons and Simple Relics III can be obtained here (in addition to number of very desirable quest reward items).
Turan
Temple of Erlik (Tier 3.5)
Enounters:
Khitai
Jade Citadel (Tier 4)
Khitai currently is the setting for the games only Tier 5 raid instance - The Jade Citadel. A high degree of competency is required. There are 7 bosses available, dropping Tier 4 Epic Armor, Accessories & Simple Relics IV.
Dragon Spine
Otherworldly Junction (Tier 5)
Tier 5 is a revisit of old bosses, though you have to enter the instance from Dragon Spine.
The Palace of Cetriss (Tier 6)
Entrance is in Dragon Spine.
This raid is currently under development, along with additional raids, and with Age of Conan Game Director Joel Bylos stating they are, "planning on releasing some other raids as a part of the Tier 6 progression."
And as regards new AA's and T6 loot, he further stated, "These two are intimately tied together and we have a solid idea of how we are going to build player progression through Tier 6 raids and new content. I’m saving all the details for a future game director letter, but new weapon combos, new items and new ways to play your favorite class are all on the table!"