There is another way that I can think of, but you may have to work with it.
Download "FL Studio" and then use the voice input to make the robotic voice say what you want. Then right click on the channel it creates and open the Piano Roll. Move the bars that indicate the pitch or the note of the sound closer to the bottom until you get the dark sound you're looking for. It may sound a little inhuman, but if you're not going to do the recording, then I don't know how else you're going to achieve the sound.
If I had a mic I could do some pretty dark sounds for you, but mine broke on me XD