Plague Warlock

Plague Warlocks are haunted survivors of the Rot Plague who agreed to bind their spirits and wills to Kasmir, the Rot. They spread the plague and tend to its monstrosities, gaining power from the ruin they spread. 
Plague warlocks are generally autonomous, but their surrender to the Rot must be genuine for the bond to form. Some are actively horrific people, some are misguided fools who think they can bend the Rot to their will, and others are just desperate survivors clinging to existence as long as possible. 
Plague warlocks are bound in their most basic spiritual essence to the Rot; even when they lose their bodies, they do not die. Instead, they linger as maligned spirits waiting to be summoned by their comrades or given ethereal form by Kasmir. Only when a warlock has completely given up the will to live do they fade into oblivion. 
While most plague warlocks are random people who either isolate themselves or cling together in small groups, some ambitious warlocks have had the brilliant idea to try and turn large numbers of other people deliberately. This has required a lot of healing magic to work, to ensure that the intended survivors actually survive the plague. Not many warlocks have this kind of power, though.
Plague warlocks and normal warlocks have an almost innate sense of rivalry. 
In considering plague warlocks, it is important to remember that the most common emotion and motivator among them is despair. Despair is what the Rot whispers first and last - despair is what justifies and enables otherwise unacceptable action.

Related Deity/Higher Power

Kasmir, the Rot