Right from the beginning, you can tell "Ligeia" definitely doesn't follow the "classic" plot arc. The narrator spends the first third of the story telling us all about Ligeia, about her origins and her beauty and her intelligence and, most of all about her intense devotion to the gal. There's lots of description and no action. You can think of it as "Ligeia: An Introduction."
Just before launching into the story of Ligeia's death, the narrator tells us, "How poignant, then, must have been the grief with which, after some years, I beheld my well-grounded expectations take wings to themselves and fly away" (8). With Ligeia's sickness and death, all of the narrator's dreams are, to put it crudely, flushed down the toilet. He's left with no hope and no guidance.
The narrator admits that he has no idea what to do without Ligeia… so it should come as no surprise when he does something regrettable.
This isn't a climax, so to speak, but it is the moment when you know things are going to get really intense. Shadows on the carpet, mysterious drops of red liquid, a sudden turn for the worse, and still pages left to go – you know that the story's not just going to end with the death of Rowena.
The first time Rowena shows signs of life, we might think to ourselves: "Man, this is weird." The second? "Huh, what's going to happen?" The eighth, ninth, tenth times? "Come on, this is getting ridiculous!" We really, really want to know what's going down.
The whole "rising from the dead" thing is amazing enough, but Poe's got another twist left in him. Sure, he shows us something unbelievable, but there's still something more to be uncovered.
Finally, we get the big reveal. Literally. You could call this the climax if you really wanted to, as this is what everything is moving towards. It's the payoff, and a big one at that.