Two diametrically opposed forces of order and chaos exist on an island outside time, reliving the same loop for an eternity.
The loop begins when the Black Rock, carrying Ricardus and his people, brings the first outsiders to paradise.
Jacob, avatar of order, maintains the skeins of history as he makes sure key figures make the right choices.
Chaos, wanting to kill Order and break the cycle, is a formless black smoke when not taking on a physical form from a deceased individual whose memories it has scanned and absorbed.
Locke came face to face with Smokey back in season one, and with his corpse back on the island Chaos finally had the loophole it needed to kill order.
"Two sides... One side is light, the other side is dark." - John Locke, season 1.