Fiends cannot be permanently destroyed, their essence always persists. As such, when the dire hyenas of the Wild Heart consumed the bodies of fallen demons belonging to the Rage of War, the lingering fiendish essence warped them into something else: gnolls. All gnolls are thus descended from fiends, and contain a fiendish spark that burns within them and drives them to violence.
Some, like those of the Znir Pact, reject their fiendish heritage and fight to control it. Others, however, embrace it or fall to it, becoming little more than wild beasts. Very rarely, there are those who cultivate it, feeding it, ultimately culminating in the hunting and consuming of a fiend. These gnolls, known in their own tongue as Vrykudlak, are the chosen mortal servants of The Wild Heart, their rampages fueled by unholy powers granted to them by the fiend they have turned into an endless battery.
Vrykudlakas are similar to lycanthropes, and they share a common origin, but their abilities are more akin to those of vampires. They can shapeshift into an animal form, like a lycanthrope, and they have an innate ability to dominate predatory animals and bend them to their will. They share the lycanthropic weakness to Silver, which is one of the few ways to meanginfully harm them, but their most distressing ability only becomes apparent when they are killed.
Killing a vrykudlak is no simple matter. When destroyed, the spirit of the vrykudlak will seek out the nearest predatory animal and possess it. The host is then driven to hunt and gorge themselves on meat, before finding a safe place to hide and enter a state of torpor. Over the course of several days, the host body is consumed to gestate a new body for the vrykudlak, which emerges fully formed in a shower of viscera and gore, ready to hunt again. The only way to permanently kill the vrykudlak is to find and kill the host animal, or purge the curse, before it can gestate.
Alternatively, like the fiends they consume, a vrykudlak's spirit can be bound to a Khyber dragonshard, imprisoning it.