The boss itself is guarded by a very short BWL-like gauntlet, which measures the raid group's ability to move and AoE as one. It also serves as an exercise in mana management, since it's very much possible to go OOM at the gauntlet and have no mana for the boss itself.
Reliquary of Souls is basically three completely different fights stringed together. Between every phase the raid is swarmed with weak ghosts, which restore your health and mana upon death. They are effectively a reset button between each phase.
First, there's the Essence of Suffering. His aura nullifies all healing and he aggroes on the closest person for 5 seconds at a time. These abilities make him an exercise in survival and movement for everyone, not just tanks. In practice, everyone tanks him for 5 seconds, and then hands off the boss to the next person. Sounds simple, but it is harder than it sounds until everyone understands how he works.
Phase 2 is Essence of Desire. She has an aura which keeps reducing your maximum mana, so naturally there's a DPS race involved. She must die before your healers have zero mana and can't prevent the tank from dying. She also has a Rune Shield which can be dispelled and a feedback ability that deals back 50% of the damage dealt. But the interesting parts are her other abilities: Deaden and Spirit Shock. Deaden increases damage on the target, so the tank must spellreflect it back to Essence of Desire to boost DPS. Spirit Shock is a quickly-cast spell which disorients the person with aggro and deals a large amount of damage. If the tank gets hit by it, Essence of Desire switches targets and casts the next Spirit Shock faster. The trick to killing her is to interrupt all Spirit Shocks while allowing the tank to reflect all Deadens and maintain maximum DPS. In other words, an exercise in total focus and seamless raid coordination.
Finally, there's Essence of Anger. He has a stacking aura which keeps dealing more and more damage as well as a timebomb-style ability called Spite. While he has a cone-like ability that burns rage as well as mana, this phase is not about tank survival per se. It's a frantic DPS race against mercilessly increasing raid damage, much like Buru phase 2 in Ruins of Ahn'Qiraj.
The combination of all these requirements in one fight makes it definitely one of the most memorable fights in World of Warcraft, because it tests all aspects of a successful raidgroup: gear, coordination, knowledge and focus. Those that can get past it get access to the next three bosses and the final 3 pieces of tier 6 sets.