Errtu

Errtu is a balor tanar'ri, and was featured as a secondary villain of the first book in the Icewind Dale Trilogy, The Crystal Shard by R. A. Salvatore, and was the main villain of Passage to Dawn, the final book in Salvatore's Legacy of the Drow series. He commands his own layer of the Abyss and the tanar'ri inhabitants who reside there.

Physical description
Errtu is a balor, the second most powerful class of the true tanar'ri (chaotic evil demons), after the demon lords. He is twelve feet tall (according to Salvatore's Siege of Darkness), covered in red scales, with a large pair of batlike wings and a tail, and is frequently described as having a horned, ape-like head with a doglike muzzle. When he's in the throes of lust for power, he drools an acidic saliva. His voice is gravelly, guttural, and harsh. He is the ruler of a layer of the Abyss characterized by the growth of giant mushrooms, and even has a throne carved into the stalk of one. His weapons are usually a many-tailed whip, a sword of lightning, and his considerable magic and wits.

Abilities
Errtu is immune to flames and his affinity to them allows him to conjure a flaming sword, whip, and ring of fire when needed.

Like all balors, Errtu emits an aura of fear and is a capable strategist.

Personality
Errtu revels in evil and chaos, nothing pleases him more than to torment others. When needed though he can be a polite and patient negotiator and is known to reward his minions.

Errtu's motivations are primarily that of the average demon's — his own power, his own survival, and the chance to cause havoc in the Prime Material Plane. He is concerned entirely with himself and his own quest for power and control, and his selfishness leads him nearly to paranoia — in his view, anything that goes wrong is a conspiracy at him. Given Errtu's native environment, and the temperament of its denizens, this is an excellent survival trait and not too far off the mark.

Despite that, Errtu is very intelligent, and a capable tactician. The simple fact that, unless something greater destroys him in the Abyss (which is unlikely), he will exist for almost eternity, allows him to have considerable patience if it will be rewarded with a considerable increase in power. He's also able to think laterally, and work around his problems. In Passage to Dawn, he released himself from a wizard's circle by using an anti-magic stone he kept in a bag, a leftover from the Time of Troubles. This dispelled the summoner's protective magic circle against evil, which was otherwise flawless, and likewise disabled Errtu's magical abilities. However, Errtu, being a balor-class demon and therefore very strong physically as well, did not need them to kill the summoner.

He has been known to cooperate with other evil beings. In The Crystal Shard he chose to ally himself with the wizard Akar Kessel, on the promise that when Kessel died (which wouldn't be long in Errtu's view, even if Kessel died of old age), he would gain Crenshinibon. In Siege of Darkness, he became a reluctant ally of the drow goddess Lolth — at first because she could obliterate him if he didn't cooperate, and later because she offered him a chance to gain total, personal revenge on Drizzt. A few times he offers subordinate posts to lesser demons to ensure their aid.

His list of enemies is quite long, with Drizzt currently his most hated one.

Errtu would make a most dangerous foe to any but the most powerful, yet for all his millennia of knowledge he is oft hampered by arrogance, impatience stemming from boredom and somewhat inflexible habits in combat.

Obsession with Drizzt
Like Artemis Entreri, Errtu holds an intense hatred of Drizzt Do'Urden, a drow outcast. Unlike Entreri, whose rivalry was based on pride and discovering who was the best, Errtu is motivated by sheer malevolence and revenge. Drizzt defeats Errtu twice in the course of his adventures.

The first time, when Errtu was serving as Akar Kessel's general, Drizzt summoned Errtu, meaning to pose as a drow scout and inquire about Crenshinibon. When Errtu discovered Drizzt's ivory unicorn pendant, he attacked the drow, surrounding himself with flames and using his whip to catch Drizzt and pull him into the flames. Upon contact with Drizzt, the flames were extinguished due to the magic of the as-yet unnamed scimitar Icingdeath. In a confused moment for both of them, Drizzt slashed Errtu with Icingdeath — causing the balor real pain and the blade's magic even freezing the tissues at the edge of the wound. Errtu and Drizzt were both shocked — having never even heard of a weapon with that effect — but Drizzt recovered first and stabbed the balor in the stomach. The scimitar's magic won out, destroying Errtu's material body and banishing him for a hundred years. As his body dissolved, Errtu reminded Drizzt of their long lifespans and promised revenge.

The second time proved much harder for Drizzt, as the balor had gained the artifact Crenshinibon, and the balor had used it to build a stronghold in the Sea of Moving Ice, a body of water west of Icewind Dale and dotted with icebergs. Despite his dislike of cold, Errtu chose the place because of its natural barriers. He opened a gate to the Abyss and let innumerable lesser fiends through, as well as more powerful demons. Drizzt and his friends managed to fight their way through, and confronted Errtu in an ice cave.

This time, Errtu had space to fly, and repeatedly teleported around the room, using his flames to melt nearby ice and fill the floor with water — in which he would periodically thrust his lightning-bladed sword to shock the companions. Drizzt and his friends would have died, if Wulfgar had not been inadvertently freed by a younger barbarian via the hammer Aegis-fang and joined the fight. Together, the companions managed to bring Errtu out of the air, where Drizzt could strike with Icingdeath and banish him for the second time.

Along with the assassin Artemis Entreri and King Obould Many-Arrows, Errtu is among the greatest nemeses and most powerful and dangerous foes of the drow renegade, though Errtu has described himself as "he who hates Drizzt most." (This is arguable, as the ranger has made many enemies in his lifetime.)

Obsession with Crenshinibon
Errtu once worked for the group of liches who created the artifact named Crenshinibon. When it was brought to "life," Errtu was cast back to the Abyss. He had thought Crenshinibon was destroyed in the same blast.

After he learned that it hadn't actually been destroyed, Errtu devoted his existence to finding Crenshinibon again. This led to altercations with Drizzt Do'Urden, who sought to keep it out of Errtu's hands. After Wulfgar was "killed" by a yochlol summoned by Vierna Do'Urden, Lolth gifted the barbarian to Errtu as a bargaining chip in exchange for his guardianship of Menzoberranzan during the Time of Troubles. His efforts eventually won out and the artifact was finally his. He struggled with Crenshinibon's dominating personality, as had all of its previous owners but was soon banished (for the second time) by a combined effort from the Companions of the Hall. Crenshinibon was destroyed by Jarlaxle Baenre not long afterward and Errtu has not been heard from on the Prime Material Plane since.

Origins
Errtu was one of the original six named "Type VI" demons in Gary Gygax's Advanced Dungeons and Dragons books. The other five were Ndulu, Ter-Soth, Alzoll, Wendonai, and Balor, the greatest of them all. The Type VI demons later took the name of Balor to refer to their breed of demon as a whole.