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.

In the future, the nameless masculine voice was later identified as Bel'akor, the Dark Master.

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 was thought that the voice is a Great Unclean One, a greater daemon of Nurgle.