Nidhogg has been said to be a creature existing before the land of Gedonelune was founded. In legends, he used to be a giant serpent that could steal magical power. Since then, he had been sealed away with the help of the Largene twins Lars and Clives. He is Clive's familiar and has a weakness for desserts. Nidhogg also has a very high and mighty attitude. 

Voice 1: Medium to deep pitched, shouty, possibly a bit annoying. A deeper pitch is more preferred due to the game lore stating Nidhogg's voice is deep, but is not required. Also open to general interpretation, as my head canon doesn't quite fit him but am leaving both options just in case

Voice 2: Deep, powerful. This voice will mainly be used when he is NOT in his reduced form. For example, we may possibly use this voice when he is in Clive's body (this part is still being though of though)

Example #1(but deeper), Example #2