The Old Iron King is a Demon-like monster soaking in a pool of molten iron in the Iron Keep in Dark Souls 2. Once a proud king to a kingdom reinforced by iron, until the kingdom sunk into the ground, leaving everyone including the king himself, to perish in the underground. He is implied to be the soul descendant of lord Gwyn.
The Old Iron King is huge, capable of knocking the player into lava with his fists. Even if you are nowhere near lava, he still hits really hard, dealing tons of damage.
The Old Iron King has the ability to breath fire, spew forth brimstone blasts and, when in half health, set his fists on fire to deal more damage and have more area of impact.
Attack His Hands
The Old Iron King is surrounded in molten iron, so the only way to fight him is to dodge his attacks and quickly swing at his hands.
The Old Iron King is weak to lightning, as such, if your weapon is imbued with lightning or rubbed down with gold pine resin, you do significantly more damage.