She saw a hall standing,
far from the sun,
in Náströnd;
its doors are northward turned,
venom-drops fall
in through its apertures:
entwined is that hall
with serpent’s backs.
She there saw wading
the sluggish streams
bloodthirsty men
and perjurers,
and him who the ear beguiles
of another’s wife.
There Nidhögg sucks
the corpses of the dead;
the wolf tears men.
Understand ye yet, or what?
...
From below the dragon
dark comes forth,
Nithhogg flying
from Nithafjoll;
The bodies of men
on his wings he bears,
The serpent bright:
but now must I sink.
from the Völuspá, by Eysteinn Björnsson
Níðhöggr is also mentioned in the poem Grímnismál. The poem identifies Níðhöggr as tearing at the tree from beneath and also mentions Ratatoskr (a squirrel) as carrying messages between Níðhöggr and the eagle who lives at the top of the tree. I couldn't find whether or not Ratatoskr and the eagle of malevolent or not. Níðhöggr certainly seems to be!
Incidently, Níðhöggr is in the computer game, Age of Mythology
