Nidhogg

A Nidhogg (Níðhöggr, i.e. Malice Striker) is a creature from Norse mythology, a dragon who gnaws at a root of the world tree. It first appeared in The Witcher: Adventure Game as an horrific opponent.

The Witcher: Adventure Game
A Nidhogg can cross your path in "The Witcher Adventure Game". This creature is very hard to defeat and one of the strongest opponents in the game (level: Gold).

To defeat it you need: Successful Sword: Receive 3 VP. Failed Shield: Suffer 1 Heart for each shield you are short.
 * Sword:5
 * Shield:4

The Witcher 3: Wild Hunt
The Nidhogg monster was originally planned as part of the RPG The Witcher 3: Wild Hunt. Geralt should be imprisoned by Yennefer on a small Skellige island called Isle of Trials.

Either you could escape by taking the eponymous trials of the island, or you could face off against a monster called Nidhogg (a name borrowed from Norse mythology, suggesting the monster was a gigantic snake of a beast) Neither this additional story part/quest nor the monster or even the isle has made it into the final game.