What is the introductory word of noun clause Do you know who died for you?

The introductory word of the noun clause "Do you know who died for you?" is who.

Here's why:

* Noun clauses function as nouns in a sentence. They can be the subject, object, or complement of a verb.

* "Who" is a relative pronoun that introduces the noun clause and acts as the object of the verb "died."

The entire noun clause "who died for you" is the object of the verb "know."