Stroke it.

You wrap your fingers firmly around the tentacle cock protruding from your pussy. It's thick, you're only just able to get your fingers around it. But you knew that already from how much it's stretching your pussy, fixed unmovingly inside it. You rub your fingers up and down the shaft, and you feel a swell of pleasure from it. It doesn't feel all that different to touching your clit, but it's more satisfying being able to grip it in your hand. You realise there's nothing stopping you from touching your clit at the same time too, so your other hand gets in on the action as well. You rub yourself while you stroke the alien cock, your thumb teasing the flared base of its head, and you can feel an orgasm building up from all of this strange sensation. The shaft throbs in your grip, and then you feel something welling up from deep inside yourself. The tentacle thickens in your cunt as something pushes through it, and that pushes you over the edge, making you cum hard around the shaft. Liquid spurts from the tip of your alien cock, over your bed.