Mobus - The Crushing Tide

Mobus <The Crushing Tide> is a level ?? rare outdoor boss with approximately 12.9 million health. He can be found patrolling around the Abyssal Breach in Vashj'ir. I have indicated his patrol area in orange on the map below. He is very large and can be spotted easily from far away.

As of patch 4.1 Mobus drops one guaranteed purple item. He drops Mobus's Dripping Halberd which is an item level 359 physical dps polearm. He also generally drops about 200 gold, another epic item and some nice jewelcrafting designs.

He is one of 5 rare outdoor bosses added in Cataclysm. There is one in each Cataclysm zone. All of these bosses are:

Akma'hat <Dirge of the Eternal Sands> - Uldum

Garr - Mount Hyjal

Julak-Doom <The Eye of Zor> - Twilight Highlands

Mobus <The Crushing Tide> - Vashj'ir

Xariona - Deepholm

In order to down Mobus you are going to need at least a tank, healer and a couple of dps, although a full group is advisable if you don't want the fight to go on forever. I also advise that all dps and heals group up together on one side of Mobus because range can easily become an issue when fighting under water. Don't stack on top of one another but stay on the same side. This will help your healers tremendously.

Mobus also has a few abilities that you should be aware of before you start the fight.

Algae - Creates a pool of algae at a target location that deals Nature damage and slows enemies. Mobus will periodically cast algae on someone in your group. It is very easy to see and avoid. It just looks like a green cloud in the water, when you see it - move.

Enrage - Physical damage dealt is increased by 10%. Attack speed increased by 30%. (lasts 2 minutes) This can be removed by classes with enrage removing abilities. If nobody can remove it the tank may need a few extra heals depending on their gear.

Ram - Ram the enemy, dealing Physical damage and knocking back enemies in a cone in front of the caster. (5 second cast) The only person that should be in any danger of getting hit my this ability is the tank. It hits pretty hard but can easily be avoided. When the tank sees Mobus begin to cast Ram he should swim down before the cast finishes and then swim back up.

Wake - Mobus' powerful tail deals Physical damage to and knocks back enemies behind him. Periodically Mobus will cast Wake which will lead to a channeled Wake ability that deals a very fair amount of damage per second to everyone around him. This is also easy to avoid. When you notice him beginning to cast Wake swim down before he begins the channeled version of the spell. For this EVERYONE needs to swim down. When the channeling is finished everyone should swim back up to their original positions. 

I've got a video below that shows the fight. This particular group in the video had 10 people in it, 1 tank, a couple of healers and the rest dps. It was definitely overkill. This fight can easily be done with 4-5 people as well, it will just take longer. The whole fight lasted about 2 and a half minutes for our group. 

He is also one of eight rare spawns in Vashj'ir. All of them are:
Abyssal Depths-

• Ghostcrawler (Spirit Beast Crab) - For lvl 85 BM hunters, can not be killed
• Mobus <The Crushing Tide> - Group Boss, drops a nice epic polearm
• Poseidus (Sea Horse) - Drops Reins of Poseidus, a bind on use underwater mount 
• Shok'sharak - Faceless one, drops some booties

Kelp'Thar Forest-

• Lady La-La - Drops a nice level 80 healer trinket

Shimmering Expanse-

• Burgy Blackheart - Drops a transformation item: Burgy Blackheart's Handsome Hat
• Captain Florence - Only killable by Horde, drops no loot
• Captain Foulwind - Only killable by Alliance, drops no loot
• Poseidus (Sea Horse) - Drops Reins of Poseidus, a bind on use underwater mount 

Update Jan 8 2012:

So I know this has been going on for a while now (I believe since 4.3 hit) and I figured I'd write a little something quick about it. I even noticed increased traffic to this particular post, probably because people are looking for an explanation as to why they are seeing their NPCScans go off half way across the world for Mobus. It seems like it's been going off for people all over Azeroth. Stormwind, Undercity, Twilight Highlands, Eastern Plaguelands, Ironforge, you name it. It seems like he can be picked up just about anywhere in all of Eastern Kingsdoms when he spawns. Check out my screenshot below, I almost peed myself when mine went off at the SW auction house because I had my volume way up.

You can even target him and see his cast bar. It's just the craziest bug. I'm sure it won't last forever either. Take advantage while you can. If you can get a group together fast enough run out there and kill him before someone else beats you to it!