ぬ

ぬ is the attributive form of ず, a classical auxiliary verb that is used for negation like ない, and connects to verbs in the same way.
As ぬ is the attributive form of ず, it may be seen modifying nouns that directly follow it. However, this use is not common in modern times, and in many cases it will be used as part of set expressions that have not changed in the time since ない became the far more widely used structure for negation. Alternatively, it may just be used for artistic expression, or to make a character seem 'classical'.

• こんなに狭(せま)い道(みち)は通(とお)れぬ。
  I cannot pass a road this narrow.
• なんだこの飯(めし)は！こんなまずそうなものは口(くち)に入(い)れぬ！
  What is this food? I will not put this disgusting thing in my mouth!
• 貴様(きさま)みたいなものには、わしなど殺(ころ)せぬ。
  I cannot be killed by someone like you.

In the case that the verb being negated is いる, the old fashion おる will be modified to おらぬ instead.

• 鈴木(すずき)殿(どの)はここにはおらぬ。
  Suzuki-dono is not here.

Alternatively, する will be negated as せぬ, and くる will be negated as こぬ. This is consistent with the standard conjugation patterns seen with the use of ず.

• あの方(かた)は人(ひと)を傷(きず)つけるようなことはせぬ。
  That person would never do anything to hurt anyone.
• 婆(ばあ)さんが川(かわ)から帰(かえ)ってこぬ。
  My wife won't come back from the river.