SPOILERS AHEAD! Please don't read unless you have read both of the books, and thoroughly at that.
Very early in the first book, before we even know who the silent innkeeper is, we hear about the Chandrian. From the story, we quickly pick up that they are some sort of evil sorcerers with eery powers. We learn some weird stuff, like fires going blue and metal rusting, but most importantly is the children's rhyme:
When the hearthfire turns to blue,This sets us up to wonder, as we do the rest of the next two books, who are the Chandrian and what is their plan?
What to do? What to do?
Run outside. Run and hide.
When his eyes are black as crow?
Where to go? Where to go?
Near and far. Here they are.
See a man without a face?
Move like ghosts from place to place.
What’s their plan? What’s their plan?
That they have a plan is a given. When they are sitting around the campfire after having slaughtered Kvothe's entire troupe, as Cinder is harassing him, Haliax interrupts to say that he thinks the rest have forgotten what their purpose is. So from Haliax himself we know that the Chandrian do have a "plan" or a "purpose"; they don't just wander around senselessly locking up legendary namers in towers or killing traveling performers.
Based on what we know of the Chandrian and the motivations given for them, I think the plan can really be one of two things:
1) destroy existence itself
2) open the Doors of Death.
There isn't a lot to go on with either of these theories: we have the attack, some rhymes, Skarpi's story and the Amyr's story, and that is really about it. Going on these, the motivation of the Chandrian (or at least Lanre/Haliax) is explicitly to destroy all existence. Lanre actually says as much himself when he binds Selitos in Skarpi's story.
Destroying existence itself is, sadly, pretty common in modern fiction. The big bad will often want to destroy every single thing, themselves included (but last), and see this as an end of itself. Sometimes writers try to give a motivation for it, but it almost isn't necessary anymore; audiences are largely willing to accept that they're trying to destroy all life because they're the bad guy and that's what bad guys do. So maybe this is what the Chandrian are going for.
And a motivation is given for why Lanre would want to destroy all existence, or as much of a motivation as we ever get. Lyra is dead. Lanre cannot get to her. Lanre cannot bring her back from death as she did for him. He has lost the only person he loves. To make it worse, he has been cursed by his efforts. As he explains his fall in to Tarbean, Kvothe lists the doors that help us deal with tragedy: sleep, forgetfulness, insanity, and death. Lanre is denied all of these; he is always awake and lucid and perfectly aware that his woman is dead and can never come back. So in reaction to this, Lanre set out to destroy all existence.
It fits the typical mold.
I don't think it's their plan.
I think their plan is actually to open up the doors to death. Kvothe listed four doors, but I think they may be more literally than they were initially given. Maybe that is what the four-plate door holds, or the Lackless door, or some other door; but doors that are not supposed to be opened seems like a very persistent theme. Maybe it is not a literal door they open, but in any case I think the gist of their plan is that they intend to somehow unite the waking world with the world of the dead.
I think this because I think Rothfuss is a good story teller, and it would be an incredible twist if we were all set for the evil monologue where the villain explains his plan and motivation for destroying the world before the hero keeps him from flipping the switch just in time... to find out that isn't what the villain wants, but that he actually logically connected ends with means (want wife back, bring wife back).
I think this because, as I said, it is a logical plan given Lanre's motivation of missing Lyra. Destroying the entire world because you can't get what you want is actually kind of crazy, and Haliax is cursedly not crazy. Destroying the entire world because you want a woman back also doesn't accomplish what you want. He wants Lyra. If he destroys the world, he still doesn't have her.
But what really tipped me off to this is that there are more Chandrian than Haliax/Lanre. Why would they want anything to do with Haliax's silly plan to destroy everything (themselves included)? Why would Cinder want to annihilate existence? He certainly seems cruel and evil, but Cinder also seems far from depressive or melancholy. Cinder seems like the sort who greatly enjoys living. He seems quite happy with murder and banditry. The old bearded man by the fire (whoever that is) seems to enjoy living. They don't seem like they all want to end their own existence.
It is much less of a stretch, though, to imagine that seven people would be absolutely driven and united in the purpose of bringing back the dead.
I don't know what the motivations of the other Chandrian might be, nor can I guess knowing so little about them.
This also more than connects with the state of the world in the frame of the story. Everything has been torn apart in the frame; the world is at war, dark creatures (technically Fae) are roaming the countryside, prices have skyrocketed, bandits are everywhere. Did the Chandrian try to open up a door to the afterlife? Did they succeed at it, or did they fail at it? Did the skrael get through because they accidentally opened a door to Fae instead. Because Fae has a backdoor through the afterlife? It accounts for the crappy state of the world; but even if it doesn't account for the state of the world, it does still fit in with it.
So that's my crazy theory: the Chandrian are trying to open the Doors of Death and bring back the dead, somehow.