She tries to stop him

In a rare moment of clarity Lisa realised she couldn't let him cum inside her. What if he could get her pregnant? What would she give birth to, some kind of abomination? In that moment she saw everything clearly, she was being fucked by some monstrous creature. Had encouraged him to, and now he was about to ruin her forever.

"No!" she barked in the most authoritative tone she could muster, trying to meet the beast's gaze as he continued to rut her with rapidly increasing fervour.

The monster seemed lost to lust however, his eyes rolled back as he savoured the sensation of finally mating his prized catch. When Lisa barked her command once more it seemed to have the reverse effect, her order sending him into a rage in his lust drunk state.

He began to fuck her more violently, pulling the head of his cock completely from her, then plunging it back in. Fortunately for Lisa he kept plumbing the same depth, not spearing deep into her guts as he well could have. All the same, he kept pulling right out and slamming back in, destroying what remained of her opening with his final four or five thrusts. He then held himself inside her, pressed against the opening of her womb and unloaded his potent seed into her. His balls pulsed and pumped, ejecting more seed than Lisa could keep inside her and it sluiced out of her broken cunt and down the monster's glorious shaft. He bellowed, a deep, triumphant sound that echoed through the valley and beyond, then slid himself from Lisa, any resistance at her entrance well and truly gone.

Fortunately for Lisa she had blacked out after the third full-blown thrust and was still out when the beast picked her up and threw her over his shoulder. He carried her back into the cave and wrapped her up in his arms before he fell into a blissful sleep. Lisa would come to to find herself changed in more ways than one, but that was for tomorrow.