In my current game, I have set the party up around The High Forest in the Sword Coast area of Faerun. There is a paladin of Mielikki
and a pair of Tiefling siblings that are hellbent on causing as much chaos as possible - to the point that the paladin player considered retiring his character in favor of a PC that fit in with the Tieflings better. I urged him to wait another game session as I had a brainstorm for an event in the prior game session that had not fully played out. That event was the discovery that Yeenoghu had ordered his gnolls to start hunting Unicorns, which is going to piss off Mielikki and lead to a pretty interesting war.
At the same time, the fiendish mother of the Tieflings contacted them with instructions to assist the paladin in his fight against Yeenoghu's forces as much as possible while also trying to corrupt him and get him to fall. She feels that weakening Yeenoghu might open a door for her to gain more power and a fallen paladin is a nice little trophy.
While I like playing players off each other, especially when they are so opposite in manner and play-style, I feel like I may have overstepped my control of the game. Originally, the main story-line was supposed to focus on the PCs saving their adopted town from gnoll raiders that would lead them into The High Forest and a Centaur/Elf conflict that had pushed the gnolls out of the area. But this story seems so much more epic. I am tempted to drop the Centaur/Elf war story as they haven't really had a chance to get into it other than seeing a few skirmish groups of Centaur.