Fathom-Lord Karathress is an apparently high ranking naga charged with the holding of broken quest NPC, broken as in the fallen Draeni that is, not a game error.  Fairly simple three add fight, nothing special to worry about but requiring of some tactics.

Abilities

Fathom Lord Karathress is accompanied by three guards, Sharkkis, Tidalvess, and Caribdis, each possessing of various class-like abilities, Hunter, Shaman, and the final one more of a mixture.

Fathom-Lord Karathress

Melee – Although somewhat low at first this increases dramatically upon gaining ‘The beast within’
Cataclysm Bolt – Deals 50% of the targets maximum HP as well as a brief stun/knockdown effect, 10 second cooldown and targeted at a random mana using player, resistable.
Blessing of the Tides – If Karathress reaches 75% while any of the three guards still live he’ll gain this buff, increasing cast time and attack speed by 66% for every add alive.

Fathom-Guard Sharkkis

855,000 HP

Melee – Moderate damage prior to using ‘Beast Within’
Multishot – 2,000 – 3,000 damage to three targets
Leeching Throw – Randomly targeted attack burning 525 mana/health per tick for 12 seconds
Summon Pet – Sharkkis will summon a pet, either the Fathom Lurker(elemental) or Fathom Sporebat(beast.)
The Beast Within – Turns Sharkkis red, as enrage, increasing his damage by 30% and his pets by 50%, lasts 18 seconds.

Fathom-Guard Tidalvess

840,000 HP

Shaman Abilities – Tidalvess will utilize many shaman abilities from shocks to windfury, etc.
Spitfire Totem –25,000 HP/1 minute duration, periodically attacks up to 5 players for 2550-3450 fire damage with a 45 yard range.
Earthbind Totem – 45 second duration, 50% movement speed debuff.
Poison Cleansing Totem – 4,000 health, removes one poison effect from Tidalvess every 5 seconds.

Fathom-Guard Caribdis

885,000 HP

Water Bolt Volley – 45 yard range AoE dealing 2775 – 3225 frost damage, resistable
Healing Wave – Will be targeted either at Caribdis or any of the other guards/boss, heals around 25-30% HP, interruptible, 15 second cooldown.
Tidal Surge – Cast every 10-15 seconds, 10 yard AoE stun for 3 seconds.
Whirlwind – Spawns a whirlwind near Caribdis that moves across the room randomly, deals no damage but incapacitates for 3-6 seconds and adds a debuff increasing cast time.

Kill Priority

One thing to be certain of is that allowing the boss to gain Blessing of the Tides is a no-no, therefore he’ll come last no-matter what order you decide to take.  All in all the shaman is fairly annoying and therefore I’d recommend he come first, the hunter will also permanently boost the bosses melee damage when killed.

1. Tidalvess
2. Sharkkis
3. Caribdis
4. Karathress

This order will eliminate the shamans high damage capabilities, then drop the hunter with the mana/life burn, and finally the priest’s heals can easily be interrupted on a steady interval making him last.

However, as each guard drops the boss will gain a new ability, the shaman giving Karathress the spitfire totem ability, Sharkkis(hunter) giving him a permanent ‘Beast Within’ buff, and Caribdis(priest) giving him tidal surge.

Another thing is to always keep DPS prepared to switch targets and drop the spitfire totem, it can easily rack up massive damage over time if left standing and contributes a lot to draining healer mana that could be better spent elsewhere.

Properly handle the guards and steadily take down the targets one at a time, this boss shouldn’t be too much trouble at all after previous challenges.