The Nightmare is a large, black, wyvern-like creature with a large column of teeth and large horizontal wings. It drops the Nightmare scale which is one of the ingredients to craft the Big Bertha. It also drops rotten flesh and the zookeeper shard.
The Nightmare spawns in the unstable dimension which is the dimension where you find floating islands. It is not particularly rare if you can fly above tree level. If you want to fight it easier, you can spawn it from the ancient dried spawn egg.
The Nightmare is a difficult boss as it can vary in sizes and damage. It is stronger than every boss except Mobzilla, The King, and The Queen.
The minimum and maximum Damage it does is 15 - 120, 11 - 18 for Defence, and 125 - 1000 Health. You can tell how much Health, Defence and Damage a Nightmare has by it's size. The smallest Nightmare is about 2 blocks long, does 15 Damage, has 11 Defence and 125 Health. While the largest and strongest Nightmare Does 120 Damage, has 18 Defence and has 1000 Health.
It is recommended to have the full ultimate set as the Nightmare deals a lot of damage. It drops Zookeeper shard('s), feather('s), item frame('s), raw beef, rotten flesh and nightmare scale(s).
The Nightmare is based of the creatures in the movie Pitch black.