Warlocks. They were like, "Eh, I don't mind swearing loyalty to this strange, otherworldly creature of possibly nefarious origin and questionable morals. I get some cool magic out of the deal." The Red Opera: Last Days of the Warlock is a new adventure and campaign setting for 5th edition from Apotheosis Studios that has a plethora of those individuals in it, all in one place. The game is up on Kickstarter now.
From the campaign:
The campaign is more than 2.5x funded with 29 days left to go.