Evil Ash is a villain in the Evil Dead movies and is none other than Ash Williams himself, or - to be more accurate - the dark-half of Ash's personality given corporeal form by the evil spirits that he has spent his life battling (known as Deadites in the series).
Evil Ash began his "life" as an entity known as Bad Ash, who appeared out of a mirror during the events of Evil Dead II and proceeded to torment his heroic counterpart - trying to drive him mad - indeed the whole Bad Ash scenario was suggestive of Ash beginning to lose his sanity and could arguably be seen as Ash battling his inner-demons on a literal level.
At any rate after a confrontation of two Bad Ash is "killed" by the original Ash and is never heard of again until the plot of Army Of Darkness - by which time Bad Ash resurrected himself as the more powerful Evil Ash, this incarnation was completely immortal and made himself the leader of the Deadite armies in direct opposition to his heroic counterpart - who was reluctantly acting as a hero to a medieval army of his own (due to time-travel related mishaps).