Balrogs are a strong and dangerous enemy, usualy met deep in the dungeon. They always wield a Fiery Greatsword. When slain, a balrog may drop one or two bits of balrog innards. They are resistant to much magic, such as Cold. They attack as fast as a skeleton and as strong as a Troll or Ogre.
Balrogs never get hungry, which makes them a good choice for polymorphing. If they eat any sort of food, they will suffer from strangulation
Avoid trying to damage them with spells, but instead use physical attacks.
Due to their default weapon, having your armour fireproofed is also highly recommended.