The Whisper was a natural ability vampires had in Blood Omen 2 to communicate telepathically. It could be considered a basic Dark Gift.
In Blood Omen 2, Umah reminded Kain of the Whisper. Presumably, this means it was an ability Kain had used before his defeat by the Sarafan Lord, and one of several things he had forgotten since that defeat.
The Whisper could be used by vampires in close proximity to guard their secrecy, or over long distances. It could not be used for reading thoughts, unbidden. Umah said it only allowed her to "speak into [Kain's] mind and hear [Kain] in return".[1,3]
Umah and Vorador used the Whisper to speak with Kain while he was travelling alone through Meridian and beyond. They offered advice and direction, helping him to progress, or find the Cabal contacts that held the information he required.
Throughout the Legacy of Kain series, Raziel never once mentioned the Whisper. It's possible that Kain's offspring missed out on this particular skill, despite it seeming pretty standard for Vorador's descendants. It's not conclusive, however. There is an arguement that wraith Raziel always uses a form of telepathic communication, perhaps a version of the Whisper. In the Material Realm, it would be impossible for him to speak clearly without a lower jaw, and in the Spectral Realm, he doesn't have a physical form, and therefore none of the anatomy required for speech. Amy Hennig (Director and storywriter of Soul Reaver, Soul Reaver 2 and Defiance) once said Raziel could talk without a lower jaw because he had 'very supple throat muscles'; it was likely a joke.
In the Playstation version of Blood Omen 2, Kain and Umah had a white aura when we saw them using the Whisper. This effect was not present in the PC version.
- [BO2] Umah (using the Whisper): "I am using the Whisper, Kain - a natural ability of vampires. In this way, we can keep in contact, even over great distances."
Kain (using the Whisper): "Yes. I remember now."
- [BO2] Umah to Kain: "Your memory has been shattered and your body has lain dormant for two hundred years. What skills you had must be recovered before you can be any use to the Cabal."
- [BO2] Umah to Kain (using the Whisper): "Never fear. I cannot read your thoughts, but only speak into your mind and hear you in return."
- A collection of interviews with Amy Hennig (and other FAQs) at the Nosgothic Realm Forums