First, the Wild Hunt are no ghost warriors. Second, they don't take an entire empire. They are not an army, only a rather small bunch of powerful evlen warriors from another world and dimension who kidnap humans to make them slaves in their own world from time to time. In the game (and after the events of the book) they search for Ciri everywhere since she has the power they want to have. Ciri can travel through time, space and dimensions and she also has the power to open a big "world-travelling door" which would enable the Wild Hunt and their race to invade other worlds with a whole army. They hate humans and see them more as animals than as respectable creatures. Ciri is the descendant of a genetically manipulated elven woman who betrayed the ambitions of the elven race (in their eyes) by having a child with a human man and fought for a reconciliation between elves and humans. Many people wanted to abuse Ciri in order to get her genes which are believed to be the only way to escape from the prophecy that said that the witcher world would end soon in an ice age. One basic story motive of the novels is that many people want a child from Ciri (sorcerers, emperors, the Wild Hunt/Aen Elle) to obtain her power to open doors between different worlds by any means necessary with Geralt and his companions (and Yenefer) trying to defend her. Some want to heal the world by doing so (or escape the prophecy) others just want more power, among them the mighty sorcerer Vilgefortz (who died in the books at the end), the Nilfgaardian emperor Emhyr (who is Ciri's real father...) and the leaders of the Aen Elle, namely Auberon - the erl king - (who died in the books), Avellac'h and Eredin, the leader of the Dearg Ruadhri, the Wild Hunt.
In the following I'll present you some passages from
The Lady of the Lake from Andrzej Sapkowski which should introduce you to the rather complex and fantastical world of the witcher. Have fun.
The leader of the riders, a black haired elf, sat on a black and brown stallion with the size of a dragon. The horse wore like every other horse of the unit a coating on which dragon scales were stitched and a demonical horned bucranium on the head. Like all elves the black haired wore a ringmail made of indredibly small rings which huddled against the body like woolen clothes under his vermilion-amaranth-carmine-red cloak.
"Avallac'" he said and saluted.
"Eredin."
"You owe me a favor. You'll pay whenever I claim it."
"I'll pay whenever you claim it."
The black haired descended. Avallac'h did the same and gave Ciri a sign to follow his example. They walked to the group of hills between low white rocks with strange forms which were vegetated with spindle treeds and dwarf-sized bushes of blooming myrtle.
Ciri observed both of them. They were evenly sized which means that both were uncommonly big. But Avallac'h's face was soft while the face of the black haired was akin to a raptor. The bright and the black she thought. Good and evil. Light and dark...
[...]
"The Aen Seidhe, the hill people, are the elves of your world. We are the Aen Elle, the erl people. And yes, Auberon Murcetach is our king"
"The erl king?"
"You could say so."
[...]
"You shall know", [Eredin] continued without waiting for an answer, "that your wild talent won't yield anything. You won't make a leap, don't even try. And even if you succeded you shall know that my Dearg Ruadhri, the red riders, can overtake you even in the abysesses of time and space."
[...]
"We, the Aen Elle, we don't care much about what your ancestor did since we had left this world a long time ago unlike the Aen Seidhe, our cousins. We had chosen another, more interesting universe. To your surprise, people could travel between worlds quite easily back then. Of course with a little talent and exercise. You undoubtfully know what I'm speaking of."
[...]
"We said to ourselves: where is the difference? We will be here and there, who really cares about if the Dh'oine [(elven word for humans)] desperately want to destroy their world and themselves? Let's go elsewehre...to another vesicle..."
[...]
"Then there was the conjunction. Even more worlds emerged. But the door is closed. Closed forever for everyone apart from a few chosen ones. But time presses. The door must be opened. Urgently.That's an imperative. You understand this term?"
"I'm not stupid"
"No, you're not." He turned his head. "You can't be. You're Aen Hen Ichaer, the elder blood. Come closer."
[...]
You are a treatening tool, a dangerous waepon. We can't allow that this weapon falls into the hands of the erl king, the fox and the sparrow hawk.
"Whom?" she stumbled "Oh,..." The fox, crevan. Avallac'h. And I know quite well who the sparrow hawk is.
The erl king is old. But the fox and the sparrow hawk can't obtain the power over the Ard Gaeth, the door of the worlds. They already obtained it once. They already lost it once. Now they can't do any more than wandering about, toddling with small steps between the worlds, alone, like ghosts, powerless. The fox to Tir ná Béa Arainne, the sparrow hawk and his riders on the Great Helix. They can't travel further, they lack the power. That's why they dream of Ard Gaeth and the power. We will show you how they already used this power once. We will show you, star eyed, once you depart from here.
"I can't depart from here. They casted a spell on me. A barrier, Geas Garadh..."
You can't be kept prisoner. You are the mistress of the worlds.
"My eye! I don't have any wild talent, I don't rule over anything. And I cut myself off from the power, in the desert, a year ago. Horsy is my voucher."
In the desert you cut yourself off from the legerdemain. You can't cut yourself off from the power which comes from your blood. You still have it. We will teach you how to use it.
"And it's not incidentally the case", she yelled, " that you want to obtain this power, the mastery of the world which I allegedly have?"
That's not the case. We don't need to obtain this power. We always had it.
[...]
The unicorn led Ciri to one of the canyons.
It's here.
"What's here?"
Descend and see.
She did like she was told to. The ground was uneven and she stumbled. Something cracked and yielded under her foot. A lightning streaked and Ciri screamed out.
She stood in a on ocean of bones.
The sandy slope of the canyon was skidded down, probably washed away by heavy rain. And it had revealed what it once covered: a giant cemetary. A field of corpses. A giant dump of bones. Of shinbones, pelvic bones, rips, thighbones. Of skulls.
She elevated one.
Another lighting and Ciri screamed out. She realized whose bones lay there. The skull which showed traces of a sword blow featured a set of teeth with canine tooths.
Now you understand, it sounded in her head.
Now you know. That's what they have done, the Aen Elle. The erl king. The fox. The sparrow hawk. This world wasn't theirs at all. It became theirs. When they conquered it. When they opened the Ard Gaeth once by betraying and exploiting us like they tried to betray and exloit you now.
Ciri compressed the skull in her hand.
"Scoundrels" she screamed into the night. "Murderers!"