The Necromancer King, Captain Arrak val Khmaw, is a former soldier and current Necromancer.
Captain Kmaw was a soldier in The Court of Swords military. After the court lost a war with The Court of Coins, Arrak returned to his home in The Riverlands to a cool reception. Angered that he was not treated as a hero for his involvement in the war, Arrak retreated to an ancient, ruined temple upriver from his home and made a pact with the forces of The Mara. Gathering up a small army of similarly disenfranchised former soldiers, Arrak dubbed himself The Necromancer King and used a demonic artifact called the Font of Dead Water to poison the river. As the Font poisoned the riverside villages and killed their inhabitants, the Necromancer King used the somewhat-unpredictable magic of the Mara to raise many into his undead servants.
With both the living and the dead at his command, the Necromancer King began conquering the Riverlands. The unnamed Fishing Village and the town of Su fan Tin were some of the first stops in his conquest, but by the time of the Time-Skip, he has conquered most of the Riverlands.