Durin's Bane is a balrog that appears in J.R.R. Tolkien's The Fellowship of the Ring, where it attacks the Fellowship as they try to pass through the halls of Moria. It is the only balrog seen and described in Lord of the Rings (though balrogs also figure in some of Tolkien's other writings, such as the Silmarillion) and thus it has become a model for balrogs in other media, including NetHack: the passage describing the encounter with Durin's Bane is used as the encyclopedia entry for "balrog" in vanilla. This is retained even in variants that add Durin's Bane.
The balrog that came to be known as "Durin's Bane" was a servant of the evil Morgoth. After Morgoth's defeat, the balrog concealed itself under the Misty Mountains, until it was disturbed or awakened by dwarves mining in their stronghold of Khazad-dûm (Moria). The dwarves gave the creature the name "Durin's Bane" after it killed their leader, Durin VI, in battle, which forced them to vacate Moria. The balrog remained in the complex after it was occupied by Sauron's orcs and trolls, and joined the occupying force in trying to kill the Fellowship of the Ring when they attempted to pass through Moria. A powerful foe, it was only defeated after a grueling fight to the death with Gandalf the Grey.
Durin's Bane is described in The Fellowship of the Ring as a vast shadowy creature with a humanoid shape and spreading "wings". (Whether these wings are physical or merely a description of its shadow is up for debate.) It wields a blazing sword in one hand and a burning whip in the other.
Vanilla NetHack makes the standard balrog one of the most powerful major demons, armed with a broadsword and bullwhip. It also has the ability to fly. In UnNetHack and dNetHack, the unique balrog Durin's Bane is modeled on the ordinary balrog, but has distinctive traits and equipment that make it an even more daunting opponent.
In UnNetHack, Durin's Bane is found in the Ruins of Moria branch, on the second level, "The Bridge of Khazad-dûm". Its attributes are almost identical to those of an ordinary balrog, but it is faster, and better equipped. It carries a wand of speed monster, a cursed potion of paralysis, an enchanted fireproof bullwhip and has a 33% chance of having a shield of reflection. It will use its bullwhip to disarm you.
... It came to the edge of the fire and the light faded as
if a cloud had bent over it. Then with a rush it leaped
the fissure. The flames roared up to greet it, and wreathed
about it; and a black smoke swirled in the air. Its streaming
mane kindled, and blazed behind it. In its right hand
was a blade like a stabbing tongue of fire; in its left it
held a whip of many thongs.
'Ai, ai!' wailed Legolas. 'A Balrog! A Balrog is come!'
(see monst.c#line3909 from the UnNetHack v5.1.0 (20131208) source code)
Durin's Bane is the quest nemesis of the dwarf noble quest (Dnb). Unlike most quest nemeses, it is not covetous and therefore does not warp to the upstair. Durin's Bane is always equipped with a cursed +9 whip, a potion of speed, and a potion of paralysis. It also has 3x the HP of most monsters of its level, so expect the battle to drag on for some time.