Sylvanas Windrunner had a very important question to answer both for herself and her people at the end of Wrath of the Lich King: "What now?" Since their emergence as an ally of the Horde, the Forsaken have sought ultimate vengeance against the being responsible for their unfortunate fate. At the end of Wrath, the Forsaken and Sylvanas attained that goal; the Lich King was dead, and the Forsaken were left with ... well, nothing really.
Sylvanas had a lot to think about as a leader, her people were looking to her for guidance and a new goal to singlemindedly march toward. What Sylvanas discovered in her pondering was that she'd forgotten about one simple fact regarding the Forsaken -- they were undead. Ever since the beginning of World of Warcraft, new Forsaken were introduced as being former soldiers of the Scourge who had broken free of the Lich King's control. With no Lich King, there was no Scourge, and with no Scourge, there was no way to bolster the Forsaken's numbers. Without new Forsaken, Sylvanas' people would quickly die out.
Please note: The following post contains spoilers for the Forsaken storylines featured in Cataclysm content. If you have not played through Silverpine, Hillsbrad Foothills or the Western Plaguelands, turn away! And go play through those zones, because they are amazing.
Garrosh asks an excellent question. As for the val'kyr, their history is tied to the Lich King just as much, if not more so, than the Forsaken's. The Lich King created the val'kyr specifically as soldiers for war from the native vrykul of Northrend. Vrykul fight for the Lich King's favor -- most notably in Jotunheim -- and the winners of those fights "ascend" via the Lich King's power into Ymirjar. Those found unworthy are transformed into scourged vargul, twisted into unsightly versions of the vrykul they once were.Garrosh Hellscream says: Get on with it, Sylvanas.
Lady Sylvanas Windrunner says: Very well, Warchief. I have solved the plight of the Forsaken!
Lady Sylvanas Windrunner says: As a race, we Forsaken are unable to procreate.
Lady Sylvanas Windrunner says: With the aid of the val'kyr, we are now able to take the corpses of the fallen and create new Forsaken.
Lady Sylvanas Windrunner says: Agatha, show the Warchief!
The Val'kyr Agatha proceeds to resurrect fallen corpses as Forsaken.
High Warlord Cromush says: ABERRATION!
Garrosh Hellscream says: What you have done here, Sylvanas ... it goes against the laws of nature. Disgusting is the only word I have to describe it.
Lady Sylvanas Windrunner says: Warchief, without these new Forsaken, my people would die out ... Our hold upon Gilneas and northern Lordaeron would crumble.
Garrosh Hellscream says: Have you given any thought to what this means, Sylvanas?
Garrosh Hellscream says: What difference is there between you and the Lich King now?
The val'kyr, on the other hand, were made primarily from the female warriors of Valkyrion. The val'kyr act as the Lich King's agents in the spirit world and judge the vrykul warriors of Jotunheim and elsewhere for worthiness. While the frost vrykul of Valkyrion deliberately seek ascension to val'kyr, the Hyldnir of Brunnhildar find the very idea disgusting and seek to wipe the women of Valkyrion out entirely for their offense to Thorim.
With the Lich King's death, the val'kyr suddenly found themselves without a master and without a way to further their existence. It's interesting because it's not without its parallels to Sylvanas and the Forsaken's fate; both groups have no way to continue to procreate or ascend after the Lich King's defeat. Sylvanas realized the Forsaken would need a way to bolster their numbers, so she approached the val'kyr for their aid.
This is why Varimathras would have had a problem with the val'kyr: they were creations of the Lich King -- the same creature that ultimately betrayed the Burning Legion -- and the val'kyr were incredibly loyal to the Lich King. As it stands, their loyalties seem to be to Sylvanas now. But who can say that the val'kyr will remain loyal? More importantly, why did they agree to ally with Sylvanas in the first place? Perhaps, being ascendants of a matriarchal society, they simply gravitated to a strong female leader after the Lich King's death. Or it's entirely possible they have other plans in mind for the Forsaken.
In Silverpine, it is revealed that the val'kyr can mass resurrect any creatures that have fallen in battle, with the exception of the worgen. Why the worgen are immune to the effects of the val'kyr hasn't really been defined; however, it may have something to do with the worgen's connection to the Emerald Dream. Regardless, there were plenty of enemies that weren't worgen -- and Sylvanas could use the val'kyr to return those Forsaken to life.
But in the end, it's not just Gilneas that Sylvanas wants. In the Western Plaguelands, the city of Andorhal is under attack by three different sources: the Scourge who once occupied it, the Alliance who seek to take it back for their own, and the Forsaken forces, who want to claim it as theirs. Leading the charge for the Horde is death knight Koltira Deathweaver, and leading the Alliance forces is Koltira's former comrade-in-arms, Thassarian. Despite the former friendship between the two leaders, after the Battle for Light's Hope Chapel, Thassarian left to join the Alliance, and Koltira joined the Horde.
The interesting part of this is Sylvanas' involvement in it all. She seems to have taken the death knights under her wing, treating them as Forsaken soldiers. Koltira is fighting for the Horde, but his loyalties during the Andorhal struggle wavered and he entered into a truce with his former friend. Meanwhile, the Forsaken continued to use the blight -- first to retake Felstone Field, which had been cultivated back to life by Alliance farmers. Felstone Field was decimated by the blight, and the dead Alliance were raised by val'kyr into new Forsaken soldiers.
Koltira Deathweaver's current whereabouts are unknown. In the meantime, Sylvanas continued her onslaught. In Hillsbrad Foothills, Sylvanas wiped out the small town of Southshore -- again, by using the Forsaken blight.Lady Sylvanas Windrunner says: I know your secret, Koltira. I'm aware of your friendship with the Alliance death knight, Thassarian.
Koltira Deathweaver says: Y... your majesty...
Lady Sylvanas Windrunner says: Arthas clearly failed when he created his death knights. You are still weak.
Lady Sylvanas Windrunner says: Fortunately, I have agents that can help you... erase this weakness. Beneath the Undercity you go.
Sylvanas opens a portal to the Undercity. A large chain lashes out and impales Koltira, dragging him towards the portal.
Koltira Deathweaver yells: No!
Lady Sylvanas Windrunner says: Shh ... quiet, death knight. When I'm finished with you, your fear will be gone.
However, not everyone is happy with Sylvanas' actions. In the Sludge Fields, Warden Stillwater is running the former human farm. Stillwater is using the former residents as experiments and trying to create a way to make more Forsaken without using val'kyr. In Stillwater's opinion, the val'kyr are an abomination, and Sylvanas should be using other methods -- like a new plague -- to create more Forsaken soldiers.
But Sylvanas' issues aren't just with rebellious servants. In addition to Stillwater, the Forsaken of Hillsbrad Foothills are under constant attack by worgen in the area. The worgen are a formidable enemy to the Forsaken of Hillsbrad, but the start of that particular conflict was sparked in Gilneas.