In a Nutshell
The Curator guards the upper reaches of Karazhan and is considered a major gear check. You need a certain amount of damage per second in order to handle the adds he spawns continuously throughout the fight. If your raid does not kill the Adds quick enough, the damage is summing up quickly, and your healers will run out of mana.
This is a phase that consists of two alternating phases. The phases bound to The Curator’s mana pool and occur in fixed intervals. Every 10 seconds, The Curator will spawn an Astral Flare that needs to be killed as quickly as possible. The summoning consumes 10% of The Curator’s mana pool. So, after 100 seconds, when he reaches 0 mana, he will enter phase 2, the evocation phase. During this phase he will do nothing and just stand there for 20 seconds. He will be taking 200% increased damage during this phase. After 20 seconds his mana will be back at 100%, and he will enter phase 1 (the flare phase) anew.
As soon as The Curator reaches 15% life, he will enter a weak enrage. This will increase his melee damage and the frequency of the Hateful Bolts. When he enrages, he will receive increased damage and will no longer spawn Astral Flares. Just bring him down quickly.
Watch out for
When The Curator enters his evocation phase, always put a Shadow Word: Pain on him to help the damage a bit. Additionally, if you are below 50% of your mana, sent your Shadowfiend in. The increased damage debuff on the curator will grant you some hefty mana regeneration (We are talking of more than 1k mana per hit!). If you are in risk of wasting mana regeneration try to put some Smiting to The Curator. Avoid the usage of Shadow Word: Death, the backlash might very well kill you.
If your tank is decently geared, you will require only one dedicated maintank healer. The remaining healer(s) should be assigned to heal the raid.
You may assign a so-called Hateful Bolt tank. This ability will be used by The Curator on any player with very high aggro and life. The exact evaluation of the target is unknown, but some offtank (may be a warlock) damaging him constantly works well. It is the job of the raid healers to keep the Hateful Bolt tank at full health.
When The Curator enrages at 15% life, concentrate your healing on the maintank and the Hateful Bolt tank. They will suffer increased damage, and loosing them in the enrage may very well lead to a wipe.