Convenient

The entity stirred, like an old fish in a deep pool. It was too tired to flee. 
“I am the Summoning Dark.” It was not, in fact, a sound, but had it been, it would have been a hiss. “Who are you?” 
“I am the Watchman.” 
“They would have killed his family!” The darkness lunged, and met resistance. “Think of the deaths they have caused! Who are you to stop me?” 
“He created me. Quis custodiet ipsos custodes? Who watches the watchmen? Me. I watch him. Always. You will not force him to murder for you.” 
“What kind of human creates his own policeman?” 
“One who fears the dark.” 
“And so he should,” said the entity, with satisfaction. 

The Summoning Dark, from Terry Pratchett’s Thud!

The art is from motoslave, who kindly sent me the original. 2 hours, and the shading wasn’t entirely comfortable. But worth it!