So.. this is part evil GM, part awesome GM.
We had a Dana Barrett crossed with a Victoria Secret model type NPC. Something evil was after her. Weird things were happening all around her, but they could never figure it out. One of the PCs had become sweet on her. PC took her as a Romantic interest. Okay. Story progresses and they finally get a clue that points out something damning. But not to kill the suspense, I pass the clue to the players who are present for the clue via GM note. PC and Dana NPC had to evacuate a scene before as PC dropped into low HP and they didn't want to risk a death. GM note: "What were you two doing while the rest of the party handled the rest of the mission? General idea." PC response: "Giggity." Okay.
Party. "Where's Migel!?" <Fast Forward to returning to the Base of Operations as fast as humanly possible>.
I set up the scene. "You come barging in, literally shouldering through the door, deep in exhausted breaths. What you feared, stares you right in the face. You see Migel, in some form of disrobed, and a very obviously naked <NPC> beside him. She smiles politely and starts to cover up." PC Migel: "The hell?!" PC Dante: "BY THE POWER OF CHRIST I COMPEL THEE!" GM: Starting an Exorcism? PC Dante: YEP! GM: Yea. It saw that coming....
GM sets another Scene. "The naked bombshell just looks at you, sadly. Almost as though she's disappointed. ...and then she starts to sputter, spit.. ...and float off her tippee toes. From the various available orifices pools and flows a black and sickly green river of ectoplasm. It flows out and behind by a few feet, pooling and beginning to rise from the floor. At first, a rippling, greasy blob, but then it starts to take form. Its form is nothing of this world, dimension or anything you've ever dreamed. Tendrils that move with uneasy grace, pulsing with a tempo that threatens to make you sick. An oblong blob of ectoplasm, tendrils seemingly randomly forming and deforming, it has mouthes randomly placed at random points of its 'body', a single cat like eye staring at you. As it moves between starring at each of you, the eye retracts into the body, only to appear at another seemingly random location. Each mouth speaks a different language with a different accent. Not all the languages you think are home to your plane of existence. It's diatribe is almost complete random gibbering but you are able to catch a few phrases. "..was fun..", "..such a sweet tasting soul...", "too late", and "They have come." ...Roll for initiative as a Tendril comes smashing down at you.