In Winter's Heart, we learn that Cyndane is most likely Lanfear, transmigrated into a new body after dying (Ch. 35, “With the Choedan Kal”). We also note from this chapter, and PoD (Ch. 12, “New Alliances”)and WH (Ch. 13, “Wonderful News”) that Cyndane is stronger in the OP than Graendal, but not as strong as Lanfear once was. Since we are nearly certain that Cyndane's body contains Lanfear's soul, what could have caused this decrease in channeling strength?

In multiple places throughout the books (e.g., tGH, Ch. 16, “In the Mirror of Darkness” & TDR, Ch. 4 , “Shadows Sleeping”), Lanfear is described as the most beautiful woman ever seen by the subject of the POV. Even Egwene thinks Lanfear is so beautiful that it makes her feel small and ugly (TDR, Ch. 25, “Questions”). The only other person in the books who elicits anything close to this sort of reaction from people is Galad, whose appearance makes women blush and have trouble speaking. But even Galad is not so beautiful as to force men to admit it; most of the men seem to be in disbelief that Galad gets such attention (TDR, Ch. 24, “Scouting and Discoveries”).

So, how likely is it that the most beautiful woman in history is also the strongest female channeler in history (perhaps one of the strongest channelers of all)? And why did this most powerful soul suddenly experience a drop in strength upon being transmigrated? This theory attempts to answer both of these questions.

We have already established Lanfear's power drop, and her place as perhaps the most beautiful woman in the world. The key to this entire theory lies in just one of Cyndane's statements in WH. When facing Alivia, Cyndane thinks,

“[Alivia] was stronger than Cyndane had been before the Aelfinn and Eelfinn held her. That was impossible! No woman could be stronger! She must have an angreal, too.” -WH, Ch. 35, “With the Choedan Kal” (Emphases from text)

Note that Cyndane does not appear to be merely arrogant about her former abilities; she actually believes that she used to be as strong as it was possible for a woman to be. If it were arrogance, she might be shocked temporarily, much as Moridin is shocked when considering Aviendha's ability to unweave a Gateway (tPoD, Ch. 2, “Unweaving”). But in this case, Cyndane immediately thinks that Alivia must have an angreal—which she does—not that these new, child-like channelers were full of surprises. The idea that a “new” generation of channelers might produce someone naturally stronger than her never crosses her mind. Why is this? Why is she so certain that she had previously reached the maximum ability for female channeers?

If we look at the text for references about Lanfear, we find evidence that she was extremely hungry for power (of any sort). Rand (speaking as Lews Therin) even remarks as such in one of their meetings. Lanfear's biggest ambition is to be the best, most powerful person in the world, even to the extent of challenging the Dark One and Creator (see more conversations with Rand). A person with this sort of motivation and ambition will naturally seek out ways to increase his or her power and influence. I believe it is entirely possible that Lanfear artificially became both the most powerful female channeler, and the most beautiful woman in the world.

As noted above, Lanfear believed that no woman could be stronger than her (and correctly deduced that Alivia had an angreal). But at the same time, something occurred that decreased her strength in the One Power when she was transmigrated. This could be because her old body was artificially enhanced, so her ability to channel is back to its normal state.

How would she have accomplished such an artificial enhancement? I can think of only a few ways.

1. The Eel'finn

We know the Eel'finn grant “wishes” to those entering their domain. Whether one must come through the Red Door ter'angreal, or the Tower of Ghenji is uncertain. We do know that Lanfear could not have traveled to their realm through the Red Door, because she was able to pass through it again with Moiraine. Perhaps, though, she had her wishes granted when visiting via a different mechanism. Truth be told, we have no idea how many Red Doors there were in the Age of Legends, nor if there were other ways to reach the realm of the Ael'finn and Eel'finn. Similarly, we know Lanfear was a researcher—could she have learned of yet another way to reach that realm?

2. Science

As I just noted, Lanfear (as Mierin) was a scientist in the Age of Legends. When she found the Dark One's prison, her team was searching for a new type of power, able to be shared by men and women. It stands to reason, then, that she had researched the One Power extensively, perhaps even exhaustively. It is possible that she discovered (or stumbled onto) a way to maximize her One Power abilities. Given her desire for exclusive power, it also makes sense that she would not share this discovery with her fellow Aes Sedai.

3. The Dark One

We really do not know the extent of the Dark One's power, so this is very speculative. It may be that Lanfear joined the Shadow and struck a bargain with the Dark One to enhance her body. This is certainly the least likely of the mechanisms, but it should be considered.