Prince Malchezaar is both an eredar prince and candidate for end/final boss of Karazhan, a spot only contested by the creature Nightbane, a boss summonable only after completing a specific quest within Karazhan.

Abilities

Infernal Summoning – Malchezaar will summon a Netherspite Infernal, the monster beginning to cast a hellfire AoE instantly and continuously upon landing.  Each Infernal lives for around 180 seconds and there is no absolute limit for how many can be standing at the same time.

Although throughout phases 1-2 the infernals will most likely number around 4 at any one time, entering phase 3 this rapidly increases as the cooldown changes from 45 seconds to 15. 

Each summoning is marked by an emote from the boss: “You face not Malchezaar alone, but the legions I command” or “all realities, all dimensions are open to me!”

Shadow Nova – Malchezaar’s Shadow Nova has a 3-second cast and deals around 3000 damage to all within 24 yards, also possesses knock-back.  This attack is unable to hit healers and ranged DPS however enfeebled melee characters must run out of the nova’s attack distance.

Enfeeble(Phase 1/2) – Enfeeble is periodically cast upon five players at random, excluding only the primary
target(hopefully the tank.)  This debuff reduces maximum health to 1 HP for the duration of 7 seconds.
Obviously while this is in effect any damage causes instantaneous death, requiring enfeebled characters to stay out of both shadow nova and infernal range for the length of the debuff.

Shadow Word: Pain(Phase 1/3) – Periodically cast upon the main target during phase one and randomly targeted during phase three, can be dispelled and deals 1500 damage a tick.

Phase 2

Thrash – Gives Malchezaar two instant attacks, similar to windfury, and is used quite unpredictably therefore capable of causing massive damage spikes at times.

Parry – Similar to the character skill, speeds up the first attack after a parry.

Sunder Armor – Just like the warrior ability, reduces armor.

Phase 3

Flying Axes – Malchezaar will unleash his axes upon a random target, functioning as a constantly strengthening attacker as their damage will rise from around 500 to over 2,000 on cloth.

Amplify Damage – Doubles all damage taken, is cast on a random target and is undispellable.

Phase One

Once phase one begins a Shadow Word: Pain will be immediately placed upon the tank as well as the timer for infernal summoning/enfeebles and shadow nova.  The basics are to do the following:

First thing is to tank the boss with your back against the wall, this prevents knockback from sending you flying.
Secondly would be keeping DPS locked in second or lower place on the aggro meter, preventing the tank from being enfeebled and therefore slaughtered/inducing a raid wipe.

Finally, phase two will begin exactly at 60%, meaning that despite the general ease and slow pace of phase one things are about to pick up quite a bit, some might even say that phase one is simply a warm-up phase, to prepare you for the coming onslaught.

Phase Two

Phase two will begin once Malchezaar reaches 60%, prepare for the real fight as he warps in axes, increasing his melee damage extensively as well as gaining parry and thrash and utilizing sunder armor.

The only thing that changes otherwise from phase one is that shadow word: pain is no-longer used, infernal summoning, enfeeble, and shadow nova will continue as before.

Thrash will be your biggest threat throughout phase 2 due to the wild damage spiking of up to/more than 10,000 in a single second.  Healers will have to essentially stagger heals, coordinating to cause a heal to go off every second regardless of the tanks current health, simply cancel the unnecessary heals and begin casting again.

The key’s to this fight are to make certain all damage/speed debuffs are present on the boss at all times, these can aid in the prevention of too much damage spiking from thrash/parry damage although not entirely negate them.

Phase Three

In this phase melee damage will return to normal, Parry, Thrash, Sunder Armor, and Enfeeble will no-longer be utilized.  Instead, the boss will begin summoning infernals at a greatly accelerated rate, covering the entire area in less than two minutes.

Another thing to watch out for is that Machlezaar will send his axes flying into the air, they will target a random player and begin attacking them for increasing damage that, although easy enough to heal through, can prove fatal if completely ignored.

Basically, this is the burn phase, destroy him before a badly-placed infernal wipes the raid, should this happen, as it likely will at least through your first attempts, just try again as you’ve passed the hardest stage, phase two, already.