Nidarc Castle can be found by entering The End again after the Enderdragon is killed and 20 Endermen are killed. Nidarc is fought in his castle.
Nidarc Castle is made out of Obsidian and has paintings of Endermen, Enderdragons and Nidarc. Endermen will spawn once you pass a painting of one Enderman, and you re-fight the Enderdragon in a flat surface with no Endermen or Ender Crystals or Obsidian after you pass a painting of an Enderdragon. Nidarc is found once all mobs in the castle are defeated, there are hidden Spawners which will spawn Creepers, Skeletons, Zombies and Spiders. Nidarc Castle has many hidden chests which hold: Diamonds, Diamond Swords, Diamond Pickaxes and a Golden Sword which may be very useful when fighting Nidarc. Herobrine paintings are in the chest rooms, Fake Herobrine will spawn in the room, to get rid of him you must destroy the painting and replace it with Netherrack, if you don't put the Netherrack, Fake Herobrine will put the room on fire, placing the painting in the chest will make Fake Herobrine never come in that room again, as the painting will be replaced with a normal painting after taking it from the chest. Nidarc is the king of the castle, he sits on his Gold and Diamond chair and holds the mighty and powerful Diamond Of Power, it looks like a blue stick with a Diamond in the middle and an Enchanted Diamond Sword which holds all enchantment abilities on the max power level.