Bad Ash/Evil Ash first appeared in Evil Dead II as a simple hallucination of Ash's, where he taunts him over dismembering "their" girlfriend with a chainsaw before proceeding to strangle him, only for Ash to snap out of it and realize he was choking himself.
In Army of Darkness, the sequel to Evil Dead 2, Ash was sucked through a time portal, where he landed in the middle of Medieval England. Striking a deal with King Arthur and his Wise man, Ash takes a horse and rides to an ancient cemetery to retrieve the evil Necronomicon, so as he can be sent to his own time, as well as allowing the people of the small kingdom to repel the evil of the emerging Deadites, whose evil has infected kingdoms as far as Ireland and Scotland. On the way to the cemetery, Ash is chased down by an unseen force, mirroring the force that first unleashed his dark half back in Evil Dead 2. In a mad rush on horseback, he hides in an abandoned windmill and stays the night so as to avoid the force as it searches for him.
However, not even a windmill would give him sanctuary. Ash's reflection started coming alive, and in a fright, he breaks the mirror the image appears in. The shattered pieces then release miniature gremlin-like versions of himself, who proceed to stab and torture him. One manages to get swallowed by Ash, and then begins to mutate out of Ash's body, first an eye. Then an arm. A whole person was budding from Ash.
With a painful pop, the clone was free from the original body. Both sides saw each other, identical in every way, save personality. The new clone, twisted like Ash was good, proceeded to slap around the good half, poking fun at him. Finally, Ash got annoyed and stuck the nozzle of his gun under the bad half's nose, blasting half off his face off and throwing him into a tree due to the impact. Ash then proceeds to drag the shattered clone body away and dismember it, systematically throwing the pieces into a grave, despite the head waking up and delivering a sinister warning. Ash then proceeded into the cemetery, where he found not one, but 3 Necronomicons. After dealing with the first 2, one which lead into another portal and the other that was a ravenous monster, he tried to recite the passage in order to allow access to the cursed book of spells.
The incantation was "Klattu Barrata Nikto."
However, Ash couldn't remember it.
So he tried to fool the book, muffling his voice of the last word so that it appeared he actually said the last part of the phrase. However, the book wasn't fooled, and in a burst of evil energy, re-animated the thousands of skeletons and corpses buried all over the grounds. At the same time, the dismembered body of Bad Ash was revived, fused and stitched back together into a patchwork Frankenstein of a monster. But now, instead of looking human, he had a bulging lower jaw ripped of flesh, glowing yellow eyes and rotted skin and spiky long black black hair, effectively infected by the evil of the Deadites and turned into something far beyond a mere human.
Evil Ash was born, and he was ready to take back what he felt was his. Almost immediately he took control of the awakening zombie hordes, making himself their general. Donning a tattered red cape and the wicked skull of a monster as a helmet, he rallied his skeleton crew in order to device a preemptive strike. And he knew exactly how to make his intentions clear.
Sending out a bat-like Deadite, Evil Ash managed to capture his good half's new girlfriend Sheila. Then, with a look like a mix between disgust and glee, he forcibly kisses her, despite his jagged teeth and crooked jaw. Through his smooch, he infects Sheila and turns her into his own Deadite princess, as a way to gain power and bragging rights over his "twin".
With his new bride and re-animated army, Evil Ash attacked King Arthur's castle, which was fortified and defended by the "good" Ash as well as others of Arthur's court. After a long, bloody battle, the humans where aided by the arrival of Duke Henry the Red, the leader of a group of northern knights whose own castles have been attacked by the evil of the Deadites.
Evil Ash, in a mad dash to retrieve the Necronomicon, ends up battling his good half, getting stabbed in the back and then impaled before being lit on fire and thrown over the ledge of the castle walls. Quickly recovering, he rises again to reveal his body is now stripped of muscle and flesh, leaving only a skeleton with bulging eyes. Ash fights his skeleton double and both fall over the wall, where Skeleton Evil Ash lands on a catapult loaded with gun powder. Ash cuts off the skeleton's hand holding the Necronomicon and launches his boney enemy into the sky, where he explodes, destroying the last of his body.
The evil of Evil Ash was seemingly destroyed alongside his Army of Darkness, but, while his body was gone, the dark soul within still lived. Plotting revenge. With the patience only a demonic entity could have.
Evil Ash returns o the Evil Dead lore through graphic novels, specifically starting with Ashes 2 Ashes #3, once again commanding an army of the dead to destroy Ash and enslave humanity. Despite being defeated again and again by Ash, Evil Ash is relentless in his goal to destroy his good "twin" and enslave humanity with his undead hordes. To this day he remains Ash's greatest foe.
In other media
Bad Ash returns as the main antagonist in the 2000 video game Evil Dead: Hail to the King, where he is once again released by the Necronomicon, and kidnaps Ash's new girlfriend Jenny to release the Dark Ones upon Earth.
In this game, unlike in the film Army of Darkness where he appears to be a rotten corpse, in this he resembles more of hero Ash when he was originally infected by the evil of the Deadites, gaining characteristic white eyes with deep eye sockets. He also lacks his armor and twisted jaws, wearing instead a more worn out set of clothes like his "good" counterpart.
Powers and abilities
Evil Ash is everything the original is, only malicious and twisted. Inheriting from hero Ash, Evil Ash is a highly skilled combatant and able to devise complex problems in order to trip up his enemies and to attack them via his army horde, much like how a high ranking military general would do.
He also has abilities derived from his Deadite nature, such as possession, immortality, re-animation, and overall invulnerability, as seen to be in a much higher level than the skeletons he commands. Evil Ash also can withstand high impact attacks, such as swords and pillars impaling him, with very little effect on him, outside of enraging the powerful zombie Deadite commander even more than necessary.
Evil Ash is unique in that he can create a Deadite out of a living human, something seen only by direct application of the Necronomicon. He successfully is able to perform this on hero Ash's love interest Sheila, turning her into an evil witch Deadite version of herself that Evil Ash refers to as his "flower" (so far only in the Army of Darkness graphic novel adaptation does he refer to her as such).
Evil Ash has also been shown to have super strength far beyond those of a standard human (an example being when he tears the cage around the Necronomicon in his drive to retrieve it for himself). He also is agile and swift when compared to most other Deadites.
While "good" Ash is knowledgeable in sciences (an example being when he made gun powder from a high school chemistry book), Evil Ash seems to prefer the magical arts (most likely he knows science like his good half, but uses magic to deviate himself from the other). The magical arts in question is to summon and re-animate zombies to do his bidding in a powerful display of necromancy.
Evil Ash is also a highly skilled tactician and leader with intense strategical prowess. He is also well versed in fighting with swords and daggers.
In the video game Evil Dead: Hail to the King, a resurrected Evil Ash, despite being significantly weaker than in Army of Darkness, is nonetheless capable of summoning fireballs and is able to transform into more monstrous forms. Whether or not he can shape shift into more conventional forms is still yet to be seen.