In a change from our typical reaction to seeing a giant dragon inhabiting a roughly boss-sized room, we actually aren't trying to kill Valithria in any way. She is one of the few inhabitants of ICC that is still in possession of free will, and the job has fallen to us to save her from captor, Arthas. This captured green dragon remains mostly passive throughout the fight, besides opening portals to the Emerald Dream realm for your healers to take. The real fight consists of a serious amount of add control: focusing down high priority targets while also making sure to keep the boss and your healers safe from attackers.
Unfortunately for us melee classes, this means that we are faced with the grunt work of handling waves of adds, using Tricks to help with threat and mob control, and a hell of a lot of running around. If you're not Mutilate, you need to go get a pair of boots and put Cat's Swiftness on them ASAP. The adds can come from both sides of Valithria's room, and your responsibility will be focusing killing the dangerous adds wherever they're spawning at.
A rogue's first DPS priority should be the Blazing Skeletons that spawn. They have a nasty AoE aura that will put way too much pressure on your healers if they're allowed to live for more than a few seconds. If you're within range to attack a Blazing Skeleton, make it your first priority. I also suggest using Tricks to misdirect these adds immediately to a tank, as the DPS classes will need to start nuking it immediately for best results. You can Feint to help your healers out if you fail to kill the add before it begins pulsing its AoE fire attack.
Assuming all Blazing Skeletons have been dealt with, your next priority will be Suppressors. These guys don't do much outside of applying a Mortal Strike-style debuff called Suppression to the boss, which will slow down your healers' progress on healing Valithria. Make sure to focus these guys down: they're a perfect candidate for Blade Flurry since they tend to come in packs and don't have a true aggro table. You can use any stuns and slows you have to stagger their approach as well.
Now that our two top priority adds are under control, the next 3 add types carry a lower urgency. Gluttonous Abominations are the strongest of the remaining adds, and are typically killed next in line. Make sure you don't stand in front of them, or you risk being sprayed with some nasty ooze. When they die, the aboms spawn a few Rot Worms which hit for a ton. Try to let your tank pick up aggro on these, and maybe help out with a Fan of Knives or two.
Our strategy had rogues avoiding the Blistering Zombies due to their Acid Burst attack, and because rogues are ridiculously good at nuking the Risen Archmages. The Archmages have a pretty simple Frostbolt Volley that they'll use to slow your raid's movement. Because this fight is so movement intensive, you'll want to kick as many of these casts as possible. They'll also drop some metaphorical fire on the ground in the form of Mana Void and Column of Frost: just stay out of stuff on the ground as usual. Feel free to Kick, Kidney Shot, or even Gouge to stop their casts while you tear them apart.
That's the entire fight in a nutshell: you'll constantly be juggling adds on both sides, swapping targets as a high priority mob spawns, and running back and forth across the room. Healers and DPS swap roles on this fight, as they tunnel vision the boss and spam their highest HPS moves to end the encounter, while rogues focus on a ton of different life bars, triaging the entire time. Obviously you will want to keep Tricks of the Trade on cooldown, as threat will be a major concern for the rest of your DPS. I try to save Cloak for a situation where I'm slowed far away from the action, but you can also use it to help counteract some of the Blazing Skeleton AoE aura if one is slow to die.
Sindragosa is a pretty simple 3-phase dragon fight, which you'll have experience with if you've ever fought Sapphiron in Naxxramas. For 90% of the fight, you'll be back to your normal dragon slaying ways. Stand on the boss' side, slightly towards the rear, at maximum range. You want to make sure you never run in front of Sindragosa, as her Cleave or Frost Breath will easily dispatch you. Running behind her is no fun either, as her tail will have no problem sweeping around to crush you. Once you're in position, you won't have much moving to do.
A key mechanic behind the Sindragosa encounter is the Chilled to the Bone debuff. This stacking DoT will do some serious damage to you as it builds up, and is considered very deadly at around 10 stacks and above. Mutilate rogues have it worse than combat on this fight, as their quicker weapons tend to cause more stacks to build up. We are luckily able to Cloak of Shadows out of the debuff, setting us up to stack our debuff higher than some of the other melee. Typically, you'll want to Cloak once you're around 8-10 stacks after starting the fight. You'll quickly start building up new stacks, but shouldn't be too high before the boss' next special move becomes active.
The boss will pull everyone towards her, similar to the Shade of Aran in Karazhan and Cyanigosa in Violet Hold. Again, sharing a mechanic with the Shade of Aran, she'll begin casting her very powerful AoE attack after you've been pulled in. As soon as you're death gripped to her, immediately run away from her. You don't have much time to avoid the attack, and so moving instantly is very important. Once her AoE has gone off, you can return to DPSing the boss. While you're running around from her, your Chilled to the Bone stack should fall off, giving you some more breathing room to begin your assault again. Your stack shouldn't get too high before she takes off for her air phase, again allowing you to clear your CttB stack.
While she's in the air, she'll target two (or five in 25m) people with her Ice Tomb ability. These are nearly perfect copies of Sapphiron's ability, including the massive damage to anyone nearby the Tombed target. Once you or your raidmates are frozen in ice, you'll use their Ice Tomb to block Sindragosa's frost bomb attack. You just stand behind the Ice Tomb, directly on the opposite side from where the bombs are going off. The bomb locations vary, so you may need to adjust your positioning between bombs. Once all four bombs go off, you'll need to break your friends out of their Ice Tombs before they asphyxiate. You'll want to be doing light DPS on whichever tomb you're behind, bringing it down to maybe 20% and then waiting for the frost bombs to finish exploding.
After a few transitions between Phases 1 and 2, you'll bring the boss down to 30%. At this point, Sindragosa takes after Sapphiron again, and stays grounded. However, she'll now start tossing out a debuff called Mystic Buffet, which will make you take increased magical damage. This can be Cloaked, and you should Cloak it when possible to reset both your CttB stack and your Mystic Buffet stacks. Realize that Mystic Buffet also increases the damage you take from CttB, so you will want to keep your CttB stack closer to 5-7 rather than 10 which would spell immediate death. You may have to stop attacking for a moment and let your CttB stack fall off if Cloak is on CD.
Your raid mates will begin getting frozen in Ice Tombs, and it's up to your raid leader to determine what your role is. If you're assigned to break Ice Tombs, make sure you are far enough away from the Tomb target before they're actually frozen, as you don't want to eat any of the AoE damage that is caused when they're frozen. Once iced, run up and blow up the Ice Tomb immediately. If you use line-of-sight via the Ice Tomb to avoid the dragon, your Mystic Buffet stacks can also fall off naturally. I assigned the melee DPS to full-time boss DPS, and so making sure CttB stays low and our Mystic Buffet stack never gets out of control become your only priorities. The boss has to die fast after reaching 35%, so be sure to have a set of cooldowns on reserve to pop once she is nearing Phase 3.
With these two dragons taken care of, only one target remains: Arthas. The Lich King will have to answer to the world for his sins, and it is our job to be the executor of his death sentence. One of the most complicated fights to date, the Arthas encounter is sure to be a favorite for those ready to fight him and destroy Frostmourne.