She is obviously in shock. Focus on the threat in front of you.

"Karen, I know its scary but I need you to reach into my pocket and use my phone to call for help." You say as confidently as you can, your eyes still locked on Alex and Gwen. Karen giggles, a movement echoed unnervingly in her tormentors, but begins to reach her right hand down towards your phone.
"Now, you two don't even think about moving or-." Your thought is broken off by Karen's wandering hand, which has reached into your underwear instead of your pocket! She gently massages your cock, her ice cold hand doing nothing to detract from the immense feeling of arousal that washes over you.
"Oh dear Linus. Not as big as I had hoped..." You feel your face flush with blood.
"But don't worry," She whispers in her sultry way. "We can fix that." She begins to massage your penis with one thumb, her other fingers wrapped around your length. She presses her self into you, wraps a leg around yours and grinds her clit into your backside. The combination of this makes you very very hard. She kisses you on the shoulder briefly, then locks her lips on your neck, her tongue feathering your goosebumped flesh tenderly. Then, ever so gently, ever so sweetly, she sinks her fangs into your neck, all the while increasing the intensity on your member. Millimetre by millimetre she invades your flesh, but this only seems to add to the intensity of her now vigorous stimulation. You can feel your warm blood gushing into her hungry mouth, but you no longer care, lost in rapture as you are. Then, in one painful stab, she bites down, hard, widening the wound and gulping heavily at the last of your lifeblood. You yelp but at the same time she finishes you off so that your cries of pain instead become gasps of pleasure. Then, one hand on the small of your back, the other still cupped comfortably around your genitals, she lays you on the ground and kneels beside you smiling. Then, with a final, tender, kiss, the darkness consumes you.