The Ardeur is the greatest of the powers in Belle Morte's line vampires. Prior to becoming an undead vampire, Belle Morte was a human psychic or "psychic vampire" known as a succubus who fed upon psychic energy related to sex and lust. Through her psychic ability, the ardeur, she was able to metaphysically feed on emotions as vampire would physically feed off of blood. When she become an undead vampire, she gained both vampire and succubus powers. These powers grew together and allowed her to create a line of vampires with powers related to sexuality. Some of those vampire inherited the versions of the ardeur (or powers similar to the ardeur). Of her line, the vampires Ligeia and Jean-Claude developed their own versions of the ardeur. When Jean-Claude entered into a triumvirate with Anita Blake and Richard Zeeman, Anita and Richard both developed their own versions of the ardeur.
Like all the powers of Belle Morte's line, the ardeur is a double-edged sword, affecting both the user and its targets. The most basic aspect of the ardeur inspires intense sexual passion in all it touches. Those under its thrall are gripped with an intense desire to have sexual intercourse with the user of the ardeur, or else the nearest person. When used lightly it is possible for a person to choose the one they have sex with, but the more powerful the ardeur becomes, the less picky its victims become (to the point where they would have sex with those they are normally not attracted to i.e. straight people would choose same-sex partners). The ardeur can be extremely addictive, making those in its grips do anything to have another taste of it. At more powerful levels the ardeur can also inspire intense love and devotion towards the wielder of the ardeur. At its most powerful, the ardeur can home in on its victims deepest desires and reshape their personalities to better accommodate those desires. It was through this that Micah and Anita found exactly what they wanted in each other as lovers, helpmates and companions.
The ardeur seems to have a primitive awareness in that it must regularly feed on the lust of its user and others. If it is not voluntarily fed, the ardeur unleashes itself without the volition of its user (often at full strength as well) and forces its user to feed it. This is a necessity as those who wield the ardeur gain power when they feed it and weaken and eventually die if they do not. If the ardeur is mastered, it can only be unleashed by the will of its wielder, but the wielder will decline in health and power if it does not feed. The ardeur can be used to feed on people's lust from a distance without the wielder's having to have intercourse with its object. This process is described as being akin to "pulling velvet through an open wound" and is thus not as pleasant as the more usual method of feeding the ardeur. Furthermore it is not nearly as effective. In The Harlequin, the ardeur is revealed to be able to feed on a master vampire or alpha lycanthrope and thus feed on anyone who has metaphysical ties to said master vampire (all its vampire underlings, human servant, animal to call etc) and alpha lycanthrope (all its pack/group etc), thus accumulating enormous amounts of power.
The ardeur drains the energies of those it feeds on and can potentially drain a person to the point of death. Thus those who possess the ardeur must have several lovers if they wish to properly feed the ardeur without killing anyone. Jean Claude and Anita can only spread the ardeur to other people (and anyone who touches said people) through physical contact. Belle Morte, who possesses the original ardeur (of which all others are but pale imitations), can spread its influence over great distances (even through dreams and metaphysical connections), affecting people even if they are not touching her. She has been said to have once used this version of the ardeur on several hundred people with only a fraction of its full power.
Known possessors of the Ardeur
Ligeia was a vampire and former lover of Requiem. Falling in love with Ligeia weaned him of his addiction to Belle Morte and her ardeur. Ligeia was the only female of Belle's line to have the ardeur, and it contained the ability to rouse lust and love both. She was nearly crazed with it initially; Requiem was with her from the beginning when it rose in her. The two of them were sent to seduce both halves of an unnamed royal couple on Belle's behalf; they were together for a considerable time on this errand. Requiem refused to have sex with Belle after that, desiring only Ligeia, and Belle was so enraged that she had Ligeia killed. (Danse Macabre)
Requiem and Anita:
Requiem: "I think that your ardeur holds not just lust, Anita, but love, as Belle's ardeur did. As my Ligeia's ardeur did."
Anita:"I've been inside Belle's head. She wouldn't know real love if it bit her on the ass."
Requiem: "She knows the ardeur as a warrior knows his weapon. She knows the art of causing love and devotion, even addiction, in others, without suffering it herself."