Found in the poisonous swamp of the Farron Keep. From the Farron Keep bonfire, turn to the right. The spell can be seen amongst some trees in the middle of the swamp. The area is comprised of deep water, though, so rolling to get it is advised.
Drastically increases physical damage absorption, resistances, and grants the player near-permanent hyper armor for the duration, but as a result, use of it substantially increases the caster's weight to the point mobility is compromised. The iron coating around the player also cripples their lightning absorption, often dropping it down far enough to where it becomes negative value (more damage is taken).
Iron Flesh grants the caster the ability to tank epic amounts of physical punishment, and since use of it also grants what would be "true poise" in most cases, they are very unlikely to be staggered about, even when being attacked by some of the heavier weapons, although chained attacks or charged attacks will still break through.
Iron Flesh should be used with great caution, as the tremendous burden it puts on the player will make it harder for them to move freely and dodge, which it most cases, may be determental to the player's survival.
One should also remain wary of lightning damage, however, and the various sources that may carry it. Casting Iron Flesh in areas where lightning is apparent will result in a quick demise, as it reduces the lightning absorption of the caster down to where they ultimately take more damage from it.