Summing up the thread.
The King is Eredin.
He was just banished in W1
After Geralt's death Ciri took Geralt and Yennifer to the place where: 1) Eredin couldn't reach them, 2) she presumed that Eredin wouldn't care about Geralt and Yennifer, herself being in a totally unreachable place. As we see in W2, Eredin DID reach Geralt and Yennifer. 2 possible reasons: Eredin found somehow some way to travel more freely or Ciri made a mistake.
What Eredin wanted from Geralt and Yennifer? He set a trap for Ciri to lure her from unreachable places back into the world of Witcher. He had no plans on killing both of them. Probably he tried to gain some information, too. When Geralt and Letho found Eredin it was a day of Midinváerne, one of the days when Red Cavalry can travel in flesh between worlds, that's why the elves were normal flesh-and-bone creatures. Understanding, that tactics of kidnapping brings nothing, Eredin let Geralt go. He moved to the plan B - tracing Geralt's and Yennifer's movements and waiting for Ciri's appearance.
First breaking point happened when Geralt met Alvin, naturally strong mage, nothing like Ciri but nevertheless interesting. After acquiring Alvin's - let's say - soul, Eredin increased his strength and started preparing to... the invasion, as shown in W3 gameplay trailer. Ships look exactly like people of Skellige imagine them - Aen Elle have tons of material to build these ships from. As usual, Aen Elle follow their proven way of operating in the Witcherworld - using fear and superstitions to prevent people from guessing their true nature. No one must understand that they are elves.
It is clearly seen that Eredin doen't share Avallac'h's view on saving all inhabitants of the Witcherworld. People are slaves for him.
We know everything Geralt experienced for the last 5,5 years, also we know that Yennifer is in Nilfgaard now. As nothing escapes his majesty, Emperor Emhyr var Emreis, Deithwen Addan yn Carn aep Morvudd, who is also Ciri's father, he is well aware about Yennifer's location.
Ciri's world on the brink of invasion, her adopted mother in the hands of her cruel biological father, her adopted father is as always risking his life -
Ciri must return and save them all. The only thing I fear - Emhyr uses Yennifer's memory loss to his advantage.
I kind of like Eredin, pity he will die.