Eygil was a magus and loyal follower of the Old Iron King. He created the pyromancies Fire Snake and Dance of Fire.
Eygil sought to grant fire a will of its own and therefore created a pyromancy that sought out the target. It is possible that he worked with the Old Iron King in bringing his iron creations to life through fire.
Eygil's idol is a massive bull and contains the final bonfire before the Old Iron King boss fight.
Eygil was originally only mentioned in the title of the third bonfire in Old Iron Keep, Eygil's Idol. The DLC adds two more descriptions of Eygil:
Fire Snake: Eygil sought to grant fire a will of its own. This pyromancy was conjured up to behave like a snake, writhing toward its prey, with a mind to strike.
Dance of Fire: Pyromancy created by the magus Eygil, loyal follower of the Old Iron King. Cast flame in a sweeping motion across a targeted area. The fire seems to dance, and makes its victims dance with it.