This fight is like Mulgar in that you have a boss and some adds to kill.
The adds have to be killed first or they give the boss a buff which means you are pretty certain to wipe!
Like the initial trash mobs in AQ when a mob here die it passes its unique skill onwards here however it is passed to the boss, meaning he is tougher and tougher each add you kill.

This fight does not require any special gear for resistance (even though there is a fair bit of magical damage), however this fight is a strong skill test.

A summary of boss and each add is as follows :

Fathom-Lord Karathress (Boss)

1,700,000 HP

Shadow Bolt

Every 10 seconds he casts a Shadow Bolt that will damage one single target for 50% of its total HP. This ability looks like five Shadow Bolts spinning in the air, but always coming down on a single target. It will cause damage equal to exactly 50% of the target's HP. It can be completely resisted, but not partially.

Enrage

After 10 minutes Fathom-Lord Karathress will enrage and one-hit everyone.

Melee Strength

At the beginning of the fight one healer is sufficient to heal the boss' tank, but once Karathress gains the buff "The Beast Within" his damage will increase enormously. He then hits the Main Tank for about 6,000 damage.

Fathom-Guard Sharkkis (Hunter)

855,000 HP

Multishot

Causes approximately 2000-3000 damage on three players.

Viper Sting

This works much like the hunter ability. It drains about 3000 mana and can not be dispelled. Do not underestimate it.

Summon Pet

Sharkkis summons a pet that is either an elemental or a beast. If it's an elemental you can Banish it easily. Otherwise it should be tanked. Rumour has it you can no longer banish the elemental.

The Beast Within

Turns red and increases damage by 30%. His pet also goes into a rage, causing 50% additional damage for 18 sec.

Melee Strength

Sharkkis deals moderate damage to the tank. It should be no problem for 1-2 healers. If his add is tanked the healers might have more problems. The pet deals at least as much damage as he does, so pay attention to your tank's HP!

Fathom-Guard Tidalvess (Shaman)

840,000 HP

Shaman Abilities

Tidalvess has nearly all the damage spells that a player shaman has, including Windfury, which is very nasty for the tank, and a Frost Shock that hits for about 6,000.

Shaman Totems

He summons several totems:

Spitfire Totem - Deals 2250-3500 damage to multiple targets in the raid. It should be destroyed as soon as it spawns.

Cleansing Totem - Can safely be ignored.

Earthbind Totem - Can safely be ignored.

Fathom-Guard Caribdis (Priest)

885,000 HP

Water Bolt Volley

This is a 45 yard area of effect spell which deals 2700-3250 damage.

Heal

A single target heal that does not depend on line of sight or range. It can be cast on any of the mobs, inluding Karathress and Caribdis himself. It heals for approximately 20-30% of the target's total HP. It can and should be interrupted by the tank, and also any melee DPS on Caribdis at the time. Ranged classes with interrupts should save them for Heals cast after Tidal Surge (see below).

Tidal Surge

Caribdis stuns all players within a 10 yard range every 15-20 seconds. They will be frozen in an ice block for 3 seconds but they still receive damage and don't lose aggro during that time. This ice block is problematic, because melee are unable to interrupt Caribdis' Heal spell during this time.

The Whirlwind

A whirlwind is generated which moves around the fight area. It has considerable range and can easily interfere with groups on the other mobs. It pushes the players up in the air for about 3-6 seconds and gives a debuff that reduces casting time. It looks like the arena whirlwind but does not deal any damage.