Arthedain

Yeah, it split.

It fractured into three separate feuding kingdoms, right? Arthedain, Cardolan, and Rudor.

Which is a strategic goldmine for the Witch King.

Because a three-way split is infinitely more toxic than a two-way split.

After ravaging Cardolan, the Witch-King lays siege to the great watchtower of Amon Sol, which we later know as Weathertop.

Right.

He burns the tower to the ground and slays King Arvelig of Arthedain.

But the surviving men of Arthedain managed to escape with the tower's most prized possession, the Palantir.