Addar is a minor Darklord from the Ravenloft setting. Originally a proud and haughty unicorn, his arrogance ultimately led to his downfall when it led to his causing the death of an elven paladin-princess who was destined to be a great champion for the powers of light, an act that caused him to be swallowed by the Mists of Ravenloft.
Addar was first mentioned as the father of the race of Shadow Unicorns in the Ravenloft Monstrous Compendium III for Advanced Dungeons & Dragons 2nd edition. He would receive a full writeup of his own in the Books of S #5: The Book of Sacrifices, making his status... sort of canon. Whilst he himself is stated as existing in the Shadow Unicorn's 3e update in Denizens of Darkness/Dread, his stats and backstory are technically fan-made.
The only canon lore of Addar is found in the RMC3, where it reads as follows: the material in the subsections below stems from his writeup in the Book of Sacrifices.
"As the Vistani tell the tale, the nightmare first appeared to the unicorn, Addar, in his dreams. Appealing to his vanity and great sense of self-importance. the nightmare claimed that there was a vast wood desperately in need of protection. Further, she said that all the denizens of that forest would be appropriately thankful and respectful of the unicorn lord were he to become their guardian – and master. Wooed by the nightmare’s dark promises and his own hubris, Addar agreed to make his way to the misty forest where the nightmare waited."
"Out of this dark alliance were born twin foals, the first shadow unicorns. It is here the tales diverge. Some say the twins slew their father, while others insist that Addar is still trapped in his darkly wooded domain, demanding worship from all who pass through his dead forest. Still others claim that the birth of the twins shattered the spell the unicorn had fallen under, and that he wanders his domain to this day driven mad by grief and self-loathing. Only the Land itself knows the truth of the matter."
On a world far removed from the Demiplane of Dread, Addar was born amongst a herd of unicorns who lived in a wide, lush forest, filled with fey creatures of all kinds, from sprites and pixies to dryads and elves. Like all his kind, Addar loved the beauty of nature and respected his sylvan neighbors, but he had one unusual failing:
Addar detested the practice amongst his herd of allowing themselves to ridden by an elven maiden. To him, it was nothing less than an utter insult, degrading a proud champion of the wilderness to the level of a mere riding beast. It didn't matter to him that the elder unicorns warned him that such thoughts were only his vanity speaking, and that the bond between unicorn and rider was a partnership of equals, not master and slave; as far as he was concerned, this was merely attempting to use pretty words like "tradition" to cover up the true meaning - servitude, and humiliation.
Such bitterness was deeply concerned to some inhabitants of the forest. Others, darker in nature, reveled in this spiritual poison. Eventually, Addar's festering pride seeped into the Lower Planes, catching the attention of a nightmare. She began visiting Addar in his dreams, using the guise of a beautiful black mare to appeal to his noble feelings, asking for help from a far-away forest that had no unicorns to protect it. Then, she began to appeal to his vanity, insinuating that the denizens of this forest would begin to worship him like the guardian spirits of his own forest.
Initially confused, Addar soon began to believe his dreams were true omens. Furthermore, he became increasingly enamored with the idea of being worshipped and revered, and so he kept silent about the dreams, letting the nightmare's sweet words fester in his once-pure soul.
This did not go unnoticed by the elder spirits of the forest. Unable to detect the cause of Addar's corruption, instead, they decided that only by binding Addar to a partner would he finally lose his wrongheaded pride and learn the wisdom they had been trying to teach him. They contacted the only princess of the local elven kingdom, a noble paladin of charming beauty and unparalleled purity, telling her Addar's name and that they wished for her to tame him. Ignorant of the greater struggles amongst the forest's spirits, she bowed to their wishes and set out for a grove deep in the forest on the next night of the full moon.
However, the forest spirits had woefully underestimated Addar's resentment of the bonding process, and how much dark strength his growing pride furnished him. Unaware of just whom she was trying to summon and bind, the elven princess repeated the incantation time and time again, and each time, Addar refused to heed the magical call. But something else did; the nightmare took advantage of the gathered magical energies and materialized in the grove, where she attacked the elven princess by setting the trees around her on fire.
The smell to smoke and the sound of terrified screams drew Addar to the grove, where he saw the princess was trapped and beginning to suffocate. He could have saved her. He chose not to. Arguing that if she were as pure and good as the forest spirits believed then she would find some way to save herself, or that another unicorn would take her as their partner, Addar turned his back and left her to her fate. As he followed the silhoutte of the black mare from his dreams into a dense bank of smoke, the last thing he saw as he looked back was the elven princess choking to death, and the last thing he thought was that nobody would ever ride him, for he was to be the king of his own forest.
And thus he damned himself. When the smoke cleared, he stood in his new prison, with only the nightmare for company. For a time, they coupled, giving birth to the first Shadow Unicorns, but ultimately the now-insane and paranoid Addar drove his progeny and his mate away.
Addar's domain is a twisted pocket-realm that slides through the mists as the Dark Powers will, normally haunting places far from the most traveled trading routes. It is a wretched realm, whose black, poisoned soil nourishes only monstrous plant-beings like evil treants, blood willows and death's head trees, with most of the non-monstrous plants being dead or sickly, and either-way full of thorns and poison. It is sparsely inahbited, mostly by predators, scavengers, and malign fey such as baobhan sith, bowray and gremishkas.
Addar's domain moves as the Dark Powers will, but is particularly drawn to the forests of Darkon and Falkovnia, where it initially resided and where his progeny have spread. Whenever it materializes, from the outside, Addar's domain takes the form of a grove of healthy, solemn and ancient trees, only revealing the true decayed state to those who foolishly venture inside. Once a victim has fallen within, only by finding a matching grove of identical mien, which lies at the very heart of the domain, can they escape - and it's rumored that this could potentially lead people out of the Demiplane, just as it reputedly lures victim in from worlds beyond the mists.
Addar keeps the valuables of whoever he encounters and subsequently slays in a great trophy-hoard, close to where the secret exit from his realm resides. This has been the doom of more than one greedy fool.
Addar himself lives in a constant state of bloodthirst, paranoia and bitterness. When not feverishly preparing for revenge-seeking agents of that long-wronged elven kingdom or against the usurpation of his own descendants, he hunts through the twisted forest, looking for people to worship and obey him, and killing those who fail to offer "proper tribute".
Standing five and a half feet tall at the shoulder and nearly nine feet long, Addar looks surprisingly pure for a fallen unicorn. His coat, beard and mane are all pure white, and the three-footlong horn sprouting from the center of his forehead is purest ivory in color. Only those who foolishly get close will spot the myriad blood-red spatter-like markings on his coat, and the malicious, crazed look in his constantly-moving bloodshot eyes.
Addar possesses most of the abilities of a unicorn, but corrupted by his darklord status, and a few abilities in common with his progeny. His horn is a +3 piercing weapon, which causes 3d12+8 damage on a charge, but he can't use his hooves on the same round that he makes a charge attack.
Like a shadow unicorn, Addar can move with absolute silence when he wishes, imposing a -6 circumstance penalty on Listen checks. However, he lacks his children's ability to become invisible. Also like them, he can generate the equivalent of a Fear spell by stamping his hooves and shrieking, but Addar's version is more powerful, mimicking the spell of a 9th level Sorcerer (DC23). He also possesses the Flaming Horn ability of a shadow unicorn, which adds +2d4 fire damage for 8 rounds and can be used 3 times per day, and enables him to carve Glyphs of Gloom.
However, his Glyphs of Gloom are more potent, causing intense melancholy in the affected area; those who fail a DC 15 Will save suffer a -2 moral penalty to all attack rolls, damage rolls, saving throws and skill checks.
He radiates a constant Magic Circle Against Good (which he can change to Magic Circle Against Evil as he wills), and can Detect Chaos whenever he wills as a free action. He can Teleport Without Error to any point within his domain 1/day, and, when covered by the shadows of his forest, he can Blink 3/day. He has lost all of the standard unicorn's curative powers, but can cast Inflict Light Wounds (3/day) and Inflict Moderate Wounds (1/day) as a 5th level druid and Poison as an 8th level druid 1/day, all by touching the victim with his horns. Finally, he's immune to all poisons, sleep, hold, charm and life-affecting magic.
Unlike some other darklords, Addar has no death-cheating powers, so if you can bring him down, he's done for.