History
In “The Door,” we learned that the mysterious Night King was created by the Children of the Forest during their war with the First Men. He appears to be immortal or close to it, and has been lying in wait for millennia for the right conditions to attack the realms of men.
The Night King first appeared in “Oathkeeper,” when he turned one of Craster’s newborn sons into either a wight or a new White Walker. His eyes turned blue, anyway. He also got a whole lot of new followers following the Massacre at Hardhome in season 5, when he raised thousands of murdered wildlings into his ever-swelling army of the dead. And just recently, he managed to destroy the previous Three-Eyed Raven, an old enemy, and is hot on the trail of the new one, Bran Stark.
Next: Sophie Turner wants Sansa Stark to have 'more kills'
Status
Winter is here. After having killed and resurrected thousands of wildlings in season 5 and nearly killing Bran in season 6, the Night King’s next logical move is to breach the Wall.
The Wall is armored with ancient magics designed to keep the White Walkers out, but the Night King has had a long time to figure out how to get around them. If he’s successful, the greatly weakened Night’s Watch won’t have a chance of stopping him. And considering that the most powerful armies in Westeros are about to cut each other to pieces down south, it seems like the perfect time for the Night King to act.