Nidhogg was a pet of the Disir. He was used in a war between the Elders and the Archons. He was later sealed in Niflheim, a shadowrealm and was imprisoned by Yggdrasil's roots.
Nidhogg is not seen in the Alchemyst but it is known that he was released from Niflheim when it and the world tree were destroyed.
Nidhogg is summoned by the Disir to defeat Scathach. He is injured by Josh wielding Clarent. Nidhogg then abducts Scatty and goes on a Rampage. He later is attacked by Nicholas Flamel, Joan of Arc, and Sophie Newman. He is defeated by Sophie Newman using Clarent. Clarent being the sword of fire, set fire to his inside and caused him to become a statue-like being.
Nidhogg is described as being a mix between a lizard/snake/crocodile and a Dinosaur. He had an alligator-like hide and a large head. He had a sticky, acid-covered, white tounge which later turned black and brown after touching Clarent.