This article contains major plot spoilers. If you do not yet know of this information, it is recommended that you read at your own risk or not at all.
The Dark Ranger
Dark Talion is the skin you unlock for Talion during the final mission of the Shadow of War storyline. Talion plays the same way as before when Celebrimbor was bonded with him, but he obtains new armor, new weapons, a new skill, wraith and Ring of Power. For instance, because Celebrimbor is gone, Talion has a new green wraith form of himself. The Wraith-World is the same, but Talion does not speak the names of Uruk or Olog captains he identifies. He talks to himself much less, seeing as there's really no one to talk to.
During The Bright Lord, Talion, Celebrimbor and Eltariel stand at the bridge leading to Sauron's fortress of Barad-dûr, leading their gathered Orc army to challenge and at last battle Sauron himself. They challenge several of the Warchiefs Sauron sends, and by the choices of the player, they dominate or kill them all. When the Warchiefs have been defeated, Isildur arrives at the bridge and battles Talion. Isildur was revealed to be a Ringwraith in the ruins of Gorgoroth. When their battle ends, Talion and Celebrimbor look into his memory.
They see Isildur in the Battle of the Last Alliance, on his knees before Sauron. Sauron, in his cocky triumph, reaches down to strangle Isildur. Before Sauron can strangle Isildur, he sees the shattered blade Narsil, previously belonging to his father, Elendil. He grabs the blade and with the hilt-end cuts off several of Sauron's fingers, one of which is the one that wears the One Ring. We see a shot of the Ring landing on the ground, and Isildur picks it up. The memory jumps to Isildur sitting on the throne of Gondor, admiring the Ring. He slowly slips it onto his finger, entering the Wraith-World. In the Wraith-World, he is invisible to the eyes of all but Sauron, whose power is diminished. Isildur stands up, eyes still on the Ring, and declares "I am the Lord of the Ring." An image of a flaming eye appears behind him, and the memory transitions to the River Anduin. Isildur is seen chopping through the Orcs that attack him, and as he flees, the Ring flies off his finger. He's shot in the back, and the Ring drops into the water.
The next memory transition leads to Orcs carrying Isildur's body on a wooden table until they eventually reach the Dark Lord himself. Sauron takes the one remaining Ring of Power, now a shadow of his physical form, and puts it on Isildur's body's finger. Isildur springs back to life, his eyes glowing orange. This is his backstory into becoming a Nazgûl.
When the backstory sequence ends, Celebrimbor eyes Isildur curiously, before quietly declaring "You are mine." The Ring on Isildur's finger slides off, and before they brand him, Talion stabs and kills Isildur, who was drained of Sauron's power. After an argument between the three of them, Celebrimbor restates something that was said by the Tower of Sauron in Shadow of Mordor: "You are merely a vessel." Celebrimbor abandons Talion and bonds with Eltariel, who takes the New Ring of Power into her possession. Celebrimbor leaves Talion to die, but Shelob talks to him through the Wraith-World and reassures him. Talion uses the last bit of his strength to pick up Isildur's Ring and put it on his finger, declaring "As long as I have breath in my body, my fate is my own."
Talion gains both a new skill-tree and physical appearance while wearing Isildur's Ring. Most noticeably is his outward appearance; his skin is now pale and drained of color, save for black veins surrounding his eyes. His eyes glow orange, and he now wears a hood. His armor and weaponry are also considerably upgraded and now called the Ringwraith legendary gear set, which includes the following:
Servant's Armor - Talion's default armor is upgraded to look more sinister and dark. The Gondorian emblem on his original armor is gone. The only colors that remain are black and dark, stained gold. He has three spikes on each bracer, wears Isildur's Ring over gloves, and boots seemingly connected to his chest plate. This armor is automatically acquired upon reaching the siege of Minas Morgul and can be upgraded.
Dark Embrace - Talion's default cloak, Ioreth's Embrace, is upgraded to look darker in color and appear tattered. Now it has a hood and reaches down nearly to Talion's ankles. Its Gondorian emblem is also gone, and it has become deep, pale purple. This cloak is acquired automatically upon reaching the siege of Minas Morgul, and it can be upgraded.
Urfael - Talion's default sword (which shared the design of the Ranger's Sword) is upgraded to look more intimidating (now sharing the design of the Sword of Horrors). It has a hilt wrapped in blue cloth, a golden pommel and guard, and the design of the blade is straighter. Oddly, in the cinematic scenes between gameplay during the siege of Minas Morgul, Urfael and Talion's dagger, Acharn, appear the way they always have. The Ringwraith form of Urfael is acquired automatically upon the defeat of Helm Hammerhand in Seregost (mission Fire and Ice), and it can be upgraded after the siege of Minas Morgul.
Acharn - Talion's son's broken sword, Acharn, gains the appearance of an actual dagger now, sharing the design of the Dagger of Horrors before upgrading. It looks similar to a Morgul Blade as we see it in the movie series by Peter Jackson; it has a bronze pommel and guard, a thin, straight blade and a black-wrapped hilt. There are still noticeable flaws in the blade, such as scratches and a few broken portions of the edges of the blade. This could just be attention to detail, or it could be a further parallel to Acharn's past as a broken sword rather than an actual dagger. This dagger is automatically acquired upon defeating Helm Hammerhand, and it can be upgraded after the siege of Minas Morgul.
Helm's Hammer - A ranged Wraith-weapon that takes the form of throwing-hammers rather than a bow. The hammer is seemingly made of iron, with a leather-wrapped pommel and an iron head with four spikes on the front and a sharp pick side. The hammer is a weapon belonging to Helm Hammerhand, and so it's acquired after Hammerhand's defeat in Seregost. It can be upgraded upon completion of the siege of Minas Morgul. It replaces Azkar, Celebrimbor and Talion's original bow (however, if you acquire the Bright Lord's Bow through Ithildin door quests, you can still use Azkar even with Helm's Hammer.)
Isildur's Ring - The Ring of Power Talion wears when Celebrimbor and Eltariel betray him and take the New Ring from him. The Ring, similar to Celebrimbor, banishes Talion from death. It helps him keep Sauron's armies within Mordor for decades before he inevitably succumbs to Sauron's power and becomes a Nazgûl. Isildur's Ring functions mostly the same as a rune, granting 182 total Focus. If Talion equips a different rune to his Ring, it will only change the amount of Focus he has; the Ring's appearance will remain the same.
Turánn - Talion, even though Celebrimbor is gone, gains a green wraith-version of Celebrimbor's Mithril smithing hammer, Turánn. In addition to being the hammer Celebrimbor used to forge the Rings of Power, Turánn becomes a weapon of war. Talion can't use it as an equipped weapon. He throws Turánn at tribal monuments should you destroy them, and he also uses it to bash Orcs that have been knocked down (if you attack them without using a ground finisher). The hammer is also a stealth weapon, occasionally used by the wraith if you perform a stealth chain. Certain throwing hammers you can acquire throughout the game also have similar designs, essentially making Turánn a ranged weapon.
Isildur's Ring also grants Talion a new ability; he can raise the dead. It is quite haunting when you remember that Talion was against the use of this power before Isildur and Zog the Eternal's defeats. At the siege of Minas Morgul, Talion automatically equips the ability. When he stands among dead Orcs, he's able to raise them from death and brand them, making them fight for him. The Orcs will still wear blue armor, but their brands will now be bright green, as well as the glow of their eyes. The same applies to branded beasts. Talion also raises the dead Gondorians, though we only see this during the siege. He can also raise dead captains and Warchiefs, although it can only be done with the Undying Loyalty add-on. Orcs (and Ologs, with a specific add-on) will also be automatically raised from the dead if stealth killed. If you are unsure about stealth killing or stealth dominating, there isn't much of a choice either way.
Talion's wraith is green and thus using his abilities looks quite different. If you have the stealth kill wraith-chain ability enabled, the wraith will be of Talion, and the sounds will be darker and frightening. When he has a full Might meter, Talion's sword will glow bright green opposed to Celebrimbor's blue. Talion's bow or throwing hammers will be green as well. The Raise Dead ability is visually similar to green Wraith Burn, an ability from Shadow of Mordor. Other than this, Talion plays the same way as when Celebrimbor was still bonded with him. He does still talk in his head. This is most notable when he fast travels to a Haedir tower, reassuring himself that he must continue his battle, and he will not give up.
Talion holds back the forces of Sauron for decades with the power of Isildur's Ring. In the final year of his freedom, Eltariel returns to see him changed, and she attempts to kill him. She fails, and Talion justifies himself, telling her that he isn't yet under Sauron's command. Eltariel resolves to join him, defending the fortresses Talion captured during the period known as the Shadow Wars. His fortress in Seregost has to be destroyed, as the rogue Nazgul took it over. When Eltariel defeats the rogue Nazgul sisters, Talion attempts to walk his way back to Minas Morgul, knowing that his time has come. Eltariel helps him dominate a Drake, and they fly back to the Gondorian fortress. There, Eltariel attempts to get Talion to give up Isildur's Ring before he falls to Sauron. However, because of her poor choice of words, Talion's fury at her betrayal drives him into his complete corruption. He declares the Ring is his, and that "Talion is no more."
Talion and Eltariel battle and Eltariel manages to kill him, though he reminds her "I am banished from death, and bound to the Flaming Eye." Eltariel witnesses his backstory, from the beginning of Shadow of Mordor to the end of the final mission of the main storyline of Shadow of War. Talion disappears into smoke, escaping with Isildur's Ring. When he returns, he waits on top of the fight pit, overlooking Minas Morgul. The Nazgul arrive for him there, and the Witch-King welcomes Talion calmly into their ranks. Talion falls and becomes a Ringwraith in full, and he joins the Nine in their hunt for the One Ring.
Talion serves Sauron unwillingly through the War of the Ring, and he's even granted his own Fell-Beast. However, when the One Ring is destroyed, Sauron's power in the rings disappears, and Talion is finally granted death. He walks out of darkness into freedom, and we see him as he makes his way to the Undying Lands, taking off his bracers and dropping Urfael and Acharn before walking into the sunrise of Valinor.