Darkeater Midir

Darkeater Midir is an optional boss in Dark Souls III: The Ringed City.

Description
A gigantic four-winged Dragon who serves as one of the guardians of the Ringed City, Darkeater Midir continues his duty to the Gods, long after their passing. His eternal battle with the dark has begun to taint him, granting him the ability to channel Abyss magic into his already destructive fire breath.

Although he remains loyal to the Gods, his time spent fighting the dark has begun to corrupt him and the Spears of the Church fear the consequences if he was ever lost completely. Shira requests that the Ashen One slays Midir before he is fully consumed by the Abyss.

Location
Darkeater Midir can be first glimpsed upon entering the city, although he will not interact with the player until they reach the bridge past the abyssal swamp. He will initially wait on the cliff for the player to approach at which point he will swoop down and breathe fire along the bridge and connecting path.

Immediately after exiting the Shared Grave bonfire, Midir will land on the bridge and block the path to the Church of Filianore. After sustaining enough damage he will fall into the chasm under the bridge. A secret passage accessed via the Ringed Inner Wall's second shortcut leads to the chasm where Midir can be fought as the DLC's optional boss.

Lore
A descendant of the Archdragons and raised by the Gods themselves, Midir was tasked with fighting against the Abyss and its creations. His immortality enables him to perform his duties without end, even after the Gods have long since perished. But his task has rendered him damaged beyond repair, as the Dark has seeped into him and infected him (his title implies that he actually consumed potent sources of Dark), and forms black crystals. It is not exactly clear whether or not he has been fully consumed, but he clearly feels the effects, able to use the Dark in combat.

Within the memories of Midir is an old remembrance of a, similar to that of Seath's moonlight power, but the memory was "from not long after the Beginning." This implies not only that Midir predates the Lords' war against the Dragons, but that he remembers a power that predates the First Flame, and even one of his fellow dragons.

Bridge ambush
When you approach the bridge after the Shared Grave bonfire, Midir will take a few moments to land allowing the player to position themselves underneath him. This will force him to rely mainly on physical attacks which can be more easily evaded than his fire breath. There is a large rock just before the bridge that the player can hide behind if they need to heal or buff a weapon.

One of Midir's most dangerous traits is his adaption to the player's strategies. If they hide behind the rock too much to evade his fire breath, he will direct it directly above them to flush them out. If they remain underneath him for a long period of time, he will fire a dark energy beam across the length of the bridge that will explode into a trail of dark flame, dealing heavy damage and often knocking the player into the abyss. Ranged weapons can be used to weaken him from the safety of the bonfire entrance but he will fly away after a few moments, forcing the player to lure him out again.

Boss encounter
When fought as a boss, Midir possesses extremely high health (behind that of Sister Friede, Demon Prince and Yhorm the Giant) and strong defenses on all areas of his body, with the exception of his head. As such, the player should focus their attacks on this area regardless of his relentless lunges and fire breath.

Whilst technically players could simply attack Midir's legs and tail, it is far too time-consuming, as it will take on average about twice the amount of hits to kill him this way, this turns an already difficult boss battle, into an endurance test that will, more often than not, result in players getting killed from attacks that Midir has to specifically counter the strategy of getting attacked in the legs and tail, such as fire breath attacks with that cover a wide area with lingering damage and a tail swipe of extreme range.

As such, players should target Midir's head, while taking certain things into consideration.
 * The difficulty of this boss fight comes mostly from Midir's sheer size constantly throwing off the camera of the game, as such, always position yourself strictly centered with Midir. This will allow you to more clearly notice his attacks.
 * Midir's attacks deal tremendous damage, but there is a hierarchy to his attacks in terms of danger: the claw/tail attacks are at the lowest, the bites are in between, the flames are very dangerous, and his dark beam attacks are beyond extremely dangerous, usually killing players with a single direct hit, even through shields. However, he only uses mostly physical attacks if you are near him which minimizes the frequency of his fire and dark attacks.
 * Always roll backwards through his lunges and regular bites, this ensures a quick dodge, and allows you to notice if he will use a fire breath attack.
 * If Midir initiates his flurry of lunges, claw attacks and breath attacks, simply roll towards him multiple times and then sprint towards him so that you can close the distance while he attacks, this will refrain him from using his deadly beam attacks.
 * If he doesn't breath fire directly at the floor, then it is safe to dodge through it. Only when he breathes fire on the floor does it mean that it will linger and deal even more damage. When he flies towards the player to bring down flames, he will always breathe the fire to the right side of the player, as such, simply run as fast as possible towards the left side and roll if possible as the flames reach you.
 * If he fires directly at the floor, this means he might use his dark beam, as such always try to stay far enough to not be damaged by the fire, yet close enough so that if he uses his dark beam you can rush to the right towards him in order to dodge the beam.
 * Once Midir has lost approximately half his health, he will start channeling the Abyss into his attacks. He will use a area of effect blast at close range that summon large humanity orbs that follow the player similar to Affinity and his charge attacks become longer, making it challenging to find an opening to counterattack. His most dangerous ability is an even deadlier dark energy beam that covers most of the arena and creates a series of explosions, dealing massive damage if struck, often killing players at full health, outright. This attack briefly wears him out and offers a good opportunity to strike, provided the player is close enough to take advantage of this.
 * Should players stagger his head, they will be able to deal massive damage to Midir. Enough take about 15% of its health
 * Has a grab attack where he tries to grab the player in his mouth lunging forward burn them and throw them away within a huge distance. He prepares this attack while emitting a great roar so its a good way to know when to dodge.
 * Using a quick weapon with high damage-per-second like the Sellsword Twinblades, combined with Gold Pine Bundle whilst running to the boss's head, is quite effective given the fact that Midir has very brief moments of opportunity for the player to damage him.

Summons

 * Shira, Knight of Filianore: Shira can be summoned to aid in this fight by accepting her request to slay the dragon. Her summon sign is located in the chapel leading into the boss arena.

Trivia

 * It is possible to see Midir sitting on one of the towers overlooking the Ringed City before fighting him on the bridge.
 * Out of all bosses, Darkeater Midir drops the most amount of souls. With all soul-boosting equipment (Covetous Silver Serpent Ring, Shield of Want, Mendicant's Staff, etc.), Midir drops 1,115,370 souls upon defeat.
 * Using either the Chameleon sorcery or a Young White Branch at the boss fight arena will transform the player into a Humanity Phantom similar to those encountered in the Chasm of the Abyss in the first game.
 * Midir is a reference to Guyra, a dragon from the King's Field series.
 * Midir's boss fight arena is believed to be the same place where the First Flame was located.
 * The water in Midir's arena has a similar aesthetic to the sludge in Aldrich's, having various bones and pieces of flesh floating in it. Those corpses will bleed, if struck with any weapon.
 * The water itself is able to be seen through, it looks as though the pool is deep, as one can see the supposed floor beneath it, meaning the player would be walking on the surface of the water itself.
 * The transition attack where Midir breathes fire and then blasts his dark beam may be a homage to Godzilla, a fictional film monster.
 * It is possible the sorcery born from his soul, Old Moonlight, is just a nod to the earlier designs of the Moonlight Greatsword, as seen in Demon's Souls. And thusly, would hold no lore value whatsoever.