Here is the full list of all 21 Nightmare achievements.
You destroyed King Jelli in the Sewers. The drinking water in the city will be healthier now.
You defeated the Venus Flytrap. Now people wandering into the cave won't get hurt.
You defeated the Mad Clown that was scaring the children.
You defeated the Stone Golem disturbing the miners in Regal Hills Mine. The miners should be able to continue mining now.
You defeated the Black Widow in the Savannah Mines.
You defeated the Poltergeist in the Umbra Mines.
You defeated the alien parasite from space that was residing in Meteor Valley. This should stop the spread of the virus.
You helped General Tao and his army secure the eastern Guard Tower. The General gave you a commendation for your bravery.
You defeated the Necromancer that was raising the dead at the Holy Grounds. This should help with the zombie problem.
You defeated Dracula, a powerful enemy of Odium.
You defeated the werewolf Skar who escaped Ancient Prison.
You defeated the Joker and have broken the Oracles curse on King Ratticus.
You have become a Master Arcanist
You have become a Master Assassin
You have become a Master Berserker
You have become a Master Cleric
You have become a Master Monk
You have become a Master Paladin
You have become a Master Sniper
You have become a Master Spellblade
The Black Lagoon is the homeland of most Vampire kind. After Dracula fell in battle by your hand, there has been a power struggle between his two sons Alucard and Vladimir. With your help, Alucard was able to take power of the Vampires.