One of the ongoing forces behind the Forsaken is the Royal Apothecary Society. This group was created by Sylvanas to develop what the Horde thought was a remedy to cure their condition. In reality, it seemed that they were working diligently on creating a new undead plague that would not only wipe out the Scourge but all other forms of life.

Yes, all forms of life. This means not just the Scarlet Crusade, but also the rest of humanity, the Alliance, the Horde and every player in the game. Everyone.

There were some suspicions regarding the Society's motives and regarding the Forsaken themselves, but by and large, Sylvanas and her people were left to their own devices. The Horde, it seemed, were clueless. It made very little sense to me as well, but the Burning Crusade attempted to expand a little more upon the Forsaken's odd activities, as well as shed some light on Sylvanas herself.

Somewhere between the fall of the Sunwell and the beginning of Burning Crusade, Sylvanas traveled to Quel'Thalas for her own reasons and ended up running into a former high elf named Dar'Khan Drathir. Dar'Khan held the distinction of being the greatest traitor known to the high elves, now blood elves -- he pledged his loyalty to Arthas and helped him slip through the Quel'Thalassian defenses. Dar'Khan was after Anveena, the human embodiment of what remained of the Sunwell -- and Sylvanas intended to stop him from doing it. While she wasn't successful, she did manage to distract him long enough for Anveena to regain her power and get rid of him. Sylvanas agreed to keep Anveena's identity a secret, not wanting Arthas to catch wind that the Sunwell he'd destroyed wasn't quite gone after all.
But why did the Dark Lady return to Quel'Thalas? Why was she concerned with matters that shouldn't have held any significance for the dead? Part of it can be attributed to wanting to forge a possible alliance with her people, but I tend to think a lot of it can be attributed to guilt. In life, Sylvanas was the protector of Quel'Thalas, and she not only failed to protect it, but also ended up being one of those responsible for destroying it. It wasn't her fault, really. She had no choice in the matter -- but it can be argued that she still holds herself responsible. Shortly before Burning Crusade, Sylvanas began offering assistance to the former high elves, offering both supplies and troops to help the elves with their task of wiping the Scourge from the once-verdant forests.

While initially tentative and unsure of her motivations, the blood elves eventually agreed to an alliance largely due to their own dwindling numbers and need for aid (and perhaps the slow realization that they were just as alone and isolated in the world as the Forsaken were). In addition to the alliance, Sylvanas went to Thrall and convinced him to take the blood elves in as allies under the Horde banner. Other than the forces in Tranquillien, the Forsaken were largely quiet for the Burning Crusade expansion, though there were rumors that Sylvanas traveled to and from Northrend either to scout for a possible attack on the Lich King or to establish a base in the north.

It wasn't until the end of Burning Crusade that the Forsaken showed up in force in Shattrath to "help" with the ongoing zombie infestation that signaled the beginning of Wrath of the Lich King. After all, who better to combat a seeming plague of undeath than the undead? The leader of the forces in Shattrath was Grand Apothecary Putress, a chief apothecary of the Royal Apothecary Society who was working on a cure for the plague that triggered the zombie event. Putress, appearing from seemingly out of nowhere, had never been seen until that point in time, and it was assumed he was just another apothecary, a clever one who had managed to work his way to the top.
This was not the case. While Sylvanas was working on negotiations with the blood elves and helping them find their place in the Horde, Varimathras was working on a campaign of his own. Supposedly loyal to the Dark Lady, Varimathras was anything but and still worked for and served the Burning Legion. Over the course of World of Warcraft, Varimathras worked in secret behind the Banshee Queen's back, gathering new allies to his cause, including several members of the Royal Apothecary Society. Including, as it turned out, Grand Apothecary Putress.

Putress was under the opinion that the Forsaken never truly received the respect they deserved, that his people were doomed to a destiny of being seen as nothing more than living corpses to be reviled and looked upon with disgust, even by their supposed allies. He eagerly took on the task of creating the new plague of undeath but soon fell in with Varimathras and began to plan a coup d'état to remove Sylvanas from her throne and forcefully take control of the Undercity. Vengeance, in Putress' mind, was not enough -- he wanted the world to bow at his feet.

The Royal Apothecary Society worked tirelessly on the new plague that was developed for use in Northrend, called the Forsaken Blight, to perfect it so that it would be effective against Arthas and his Scourge ... and any living that happened to get in the way. Once completed, the blight was unleashed at Angrathar the Wrathgate, not only against Arthas but also against the combined forces of the Alliance and the Horde. It was only the efforts of the red dragonflight, whose flames neutralized the blight, that prevented it from spreading further and annihilating the living for good.
Meanwhile in the Undercity, Varimathras launched the coup, executing anyone that would not join his regime and nearly killing Sylvanas. The Forsaken fled, losing control of the Undercity, and it was up to the Horde to kill Varimathras and help Sylvanas get it back. At the same time, King Varian Wrynn, who was enraged at the loss of his closest friend, Highlord Bolvar Fordragon, at the Wrathgate attack, staged his own attack on the Undercity to get rid of Putress and reclaim Lordaeron for the Alliance. Each force succeeded in their respective missions and then clashed against each other in a battle that was only stopped when Jaina Proudmoore teleported the Alliance out of the Undercity and away from harm. But the damage had been done. Despite Thrall's efforts to convince King Wrynn that the events of the Wrathgate were not sanctioned by the Horde, his explanations fell on deaf ears.

The attack was over, and while Sylvanas was in control once more, her abomination guards, which were created by the Royal Apothecary Society, were quickly replaced with the orcs of the Kor'kron Guard in an effort to keep a closer eye on the Forsaken. The Kor'kron are incredibly rude to undead players that speak to them:

• I bet you rotters thought you were pretty clever at the Wrath Gate, didn't you? Playtime's over. We're here to make sure you don't try anything.
• What's wrong, bonebag? Looking for someone else? I'm all you've got now, so if you want something, make it quick.
• I'll tell you what you need to know, but I don't have to like it. We're watching you and your kind.
• Doctor Herbert Halsey is who you want. Just don't try anything funny. Any more of that plague nonsense and you'll find an axe in uncomfortable places. (when asking where the alchemy trainer is located)

However, Sylvanas seemed largely unconcerned, her focus being turned to something much larger: the eradication of the Lich King. She moved from the Undercity to Northrend, to lead an assault on Icecrown Citadel and hopefully destroy Arthas himself. After facing off with Arthas in single combat, Sylvanas realized that the Lich King's powers had increased so much that defeating him alone was impossible.
It isn't until a group defeats Arthas with a player wielding Shadowmourne that we hear more from the Banshee Queen. Arthas drops a Sealed Chest that contains items that are to be returned to their owners, one of the items belonging to Sylvanas herself. Upon turning in the quest, Sylvanas has the following to say:

So, it is done. I had not dared to trust my senses. Too many times has the Lich King made me to be a fool. Finally, he has been made to pay for the atrocities he imposed upon my people. May Azeroth never fail to remember the horrible price we paid for our weakness, for our pride. But what now, Hero? What of those freed from his grasp but still shackled to their mortal coils? Leave me. I have much to ponder.

What now, indeed. There are still several questions that remain unanswered surrounding the Forsaken, and there are several theories that could arguably be in play:

Sylvanas' redemption Sylvanas, over time, has begun to grow a little more heart. The addition and acceptance of the Forsaken to the Horde has softened her perspective on the living. The help from the Horde during the Undercity's capture and more notably in achieving the vengeance against Arthas that drove her all these years may have also had a hand in convincing her that an alliance with the living is not only possible but preferable to simply killing them all. The death of Arthas quieted the vengeance that burned in her heart, and now it is possible for her to approach the days ahead with hope for her people's ultimate acceptance by society.

Sylvanas' vengeance Sylvanas, possessing an incredibly shrewd and tactical mind, allied with the blood elves in the hopes of increasing the Forsaken's numbers as the blood elves slowly died out from magic withdrawal. However, the Horde was still breathing down her neck, and in addition, Varimathras was proving a more irritating presence by the day. To this end, she turned a blind eye to his machinations, fully aware that he was staging a coup -- and when he "took over" the Undercity, she quietly allowed it to happen. After all, not only would she be getting rid of a thorn in her side, but she'd look very much the helpless victim to her Horde "allies,' and the "betrayal" would absolve her of any involvement with the Forsaken Blight at the same time. With the death of Arthas, she and her people can now turn their attentions on bolstering their numbers, one corpse at a time -- as soon as they get rid of the pesky Kor'kron littering their halls.
Sylvanas' grief Sylvanas traveled to Quel'Thalas for no particular reason but stumbled upon not only the traitor responsible for Arthas' advance into Silvermoon, but the person responsible for her inevitable death at Arthas' hands. If Dar'Khan had not interfered, perhaps she could have stayed the assault on Quel'Thalas and saved her people from their abandonment. With the revelation that the Sunwell was not lost entirely, Sylvanas found herself strangely energized and overcome with the urge to reconnect with her living roots. After Anveena had safely been hidden away again, Sylvanas approached her former people to try and atone for her failure as a ranger-general and as a guardian of Silvermoon. The tentative alliance has convinced her that perhaps while it isn't possible to go back to her living life entirely, it is still possible for her to be accepted -- and in turn, perhaps it is possible for the Forsaken themselves to be accepted by the living. The death of Arthas was not only a gift to the Forsaken but to her former people, and now she can concentrate on leading her people to acceptance and ultimately, forgiveness.

Sylvanas' plight Sylvanas has no idea what's going on. She had a very solid reason for leading the Forsaken -- revenge against the man that tortured both her and the others she leads, the creature that cursed them to an un-life of grief and regret. But her people, who were at first so loyal to her cause, have turned against her, and with the one reason for her people's survival now taken care of, Sylvanas is left to deal with the aftermath of that betrayal. Without vengeance, what does Sylvanas have to drive her? Without the support of her people, how will she lead them to anything greater? And were the Forsaken working against her completely destroyed, or are there others that seek to undermine her rule? What are Sylvanas and the Forsaken as a whole supposed to do now?

The Forsaken are at a shaky crossroads, politically speaking: the trust they earned by working with the Horde was all but obliterated by the events of the Wrathgate. The Dark Lady seemed to have an iron grasp over her people but evidently underestimated those in her midst and lost control, leading to her flight from the Undercity. It can be argued that her loss of control is a major playing factor in current tensions between the Horde and the Alliance, which is why the Kor'kron are now watching the Forsaken so closely. Thankfully, Warchief Thrall is a decent sort who can definitely identify with dissension in the ranks and the merits of beating a tactical retreat. He wouldn't hold something like a temporary loss of control against her, much less view it as an act of cowardice.
Oh wait.

With imminent approach of Cataclysm, the Forsaken are the Horde faction that raise the most questions. With the quest for vengeance now over, will the Forsaken begin to work more closely with the Horde and try and atone for the mistakes made during the Wrathgate? Will we see the Forsaken splinter even further away from the core of the Horde and turn their efforts once more to eradicating the living? Will Sylvanas find a new cause for her people to rally under, or with the absence of Arthas, will she simply fall apart like Maiev, a huntress who is nothing without the hunt? Will the Forsaken find themselves accepted or even more ostracized than before -- and if the latter, how will they react? Will their tentative alliance with the Blood Elves strengthen, or will the restoration of the Sunwell remove any reason for association with the Forsaken? Most importantly -- what are they up to now?

Cataclysm should address at least some of these questions. Hopefully with the introduction of Gilneas and the worgen to the Alliance, the Forsaken will find themselves with new enemies to butt heads with instead of turning on their own -- but only time and the new expansion will tell.