Key to the iron door of the Iron Keep.
The Old Iron King's castle sunk into a lake
of fire, weighed down by the castle's iron,
and the burden of the king's conceit.
Over the ages, the iron was stripped from
the castle by opportunistic passers-by.
The iron door, too, must be somewhere, far away.
Located very early on in the Iron Keep.
It is the loot under the fire breathing statue past the second pair of Alonne Knights. To access it you must douse the flame or pull the lever located at the Eygil's Idol bonfire, which is the third bonfire in Iron Keep.
Alternatively you can just grab the key, but you will mostly likely die from the fire damage.