Nobody knows when he will die. The underlined clause is a/an-

A noun clause is a subordinate clause used as a noun in the sentence.
A noun clause may be used as a subject or direct object of the verb, as a predicate noun, as object of the preposition, or as an appositive.
In the given sentence, the clause when he will die serves as the object of the verb ’know’.
So, it is a Noun clause.