Nameless Voice

In the final Curse of Drachenfels mission, as well as in the pilgrimage chamber just before starting expeditions to the Chaos Wastes, a masculine voice identified by the subtitles as Nameless Voice will speak to the heroes. It is a Chaos-aligned entity that tries to get them to worship the Dark Gods and appears to be able to read their memories. His lines only exist in each characters' heads; no one else will hear what the voice is saying to them and instead express confusion when another character apparently responds to nothing in particular.

Bardin
In Castle Drachenfels:

In the pilgrimage chamber:

Kerillian
In Castle Drachenfels:

In the pilgrimage chamber:

Markus
In Castle Drachenfels:

In the pilgrimage chamber:

Sienna
In Castle Drachenfels:

In the pilgrimage chamber:

Victor
In Castle Drachenfels:

In the pilgrimage chamber:

Trivia

 * The file names for all of his lines have "daemon_whispers" in them, and the heroes' responses all have "archdaemon_response" in theirs. Considering this, and the fact that it only begins speaking to the heroes when Nurgloth the Eternal begins his daemon-summoning his ritual, it is likely that the voice is a Great Unclean One, a greater daemon of Nurgle.