Serigos was once a noble blue dragon, and went to face Yogg-Saron when it became obvious that he could control events even while locked within Ulduar. The two fought against each other, but soon Yogg-Saron bested Serigos and drove him insane. Even with Serigos insane, however, Yogg-Saron could not control him, so he locked him away in an underground cave.
Serigos, already mad, could not hold in his power, and soon he absorbed everything that kept him jailed; cold, darkness. Soon he broke free of the sealed prison, but at the cost of his physical form. Though more powerful than ever, his mind was weak and Yogg-Saron quickly took control.
Yogg-Saron sent three servants to craft a suit of armor that would keep Serigos' energy together. Though this succeeded, Serigos 'thanked' his saviors by turning them into embodiments of pure magic like himself. His former friends of the Blue Dragonfight now seek to destroy him, and aid players by turning them invisible at the beginning of the battle, aiding more than can possibly be imagined.
[Arcane Explosion]—Causes an explosion of arcane magic around the caster, causing damage to all targets within 10 yards and bringing enemies out of stealth.
[Coldstorm]— Forms a massive storm of ice below the caster's feet, dealing damage to anyone within 10 yards of the caster and freezing the ground, forming Ice patches.
[Cold Reveal]— Breaks the stealth of anyone standing on top of an ice patch.
[Arcane Beam]—Shoots a beam of arcane magic in front of the caster, dealing arcane damage to everyone (friend or foe) within the beam and breaking stealth.
During each phase, Serigos is randomly either arcane, frost, or shadow. This changes each phase, and doesn't mean much except what type of damage he will deal.
You enter the room stealthed. Serigos, sensing your presence, calls upon his servants (Arcanus, Freeze, Shade) one at a time, and they hover over the ground, casting anti-stealth abilities. The anti-stealth abilities must be avoided, as the phase automatically ends if a single player breaks stealth, because Serigos will un-stealth the others the moment this happens. Serigos calls on one servant every 5 seconds until all three servants are up.
Serigos' beam moves and is always in front of him. Serigos turns randomly in the first phase, making the beam move with him.
Once the Final servant enters the room, the raid will get a beneficial debuff called Power Overload, increasing all damage done by 50% per stack. This stacks every 5 seconds and is the only way to kill Serigos within the time limit, so let it stack to full (10). Once it reaches 10 stacks, one pre-appointed player should end the phase by stepping onto an ice patch, revealing himself and letting Serigos reveal the rest.
You face the servants, while Serigos still casts a constant, moving Beam. The beam however, now turns clockwise at a set pace and is easier to follow. The servants are identical, having an AoE and a spell that summons acolytes (non-elite humanoids). The only difference is they are immune to their own type of damage and only deal this type of damage as well. A good idea is to move them into Serigos' beam if they are not immune to the type of damage, and all dps concentrate on the one that is immune to the type of damage that the beam deals.
Serigos faces you himself, gaining two new spells, a wall and a volley spell. This phase is like the Heigan Dance, because players cannot move through the walls, have to dodge the beams (there is one for each section (sections are areas between two walls)) and must move around to avoid the volley spell.
You have 10 minutes from the beginning of phase 1 to defeat Serigos before he enrages and one-shots the raid.