The needed lubrication and some more.

The tentacles hold your thighs to provide the leverage it needs in order to penetrate you, but that doesn't stop your legs from stretching towards the ground as the pleasure fills your mind. It fucks you for less than a five minutes before speeding up at it approaches its own limit.

The substance has burned your skin the whole time, making it thinner and more sensitive, almost as if it had burned your ability to feel pain, leaving only pleasure - you don't even feel it burning anymore. The changes seem to have happened on the outside, where the substance spills on, but seems to have focused inside your ass. Every thrust into your hole makes your bottom shake with pleasure and your legs push you forward, it doesn't take more than a few thrusts to make your gasps turn into moans.

After a few more minutes of monotonous but steady assfucking, the creature stops for a moment before continuing even faster and rougher, and this send you over the edge. Your anal orgasm begins so suddenly you take a few seconds to notice what's happening, but when you do, your legs tremble and your eyes roll as the strong sensation momentarily washes all rationality from your mind. Your thighs and knees shake, extending against the ground and forcing you forward.

The combination of your new struggles and the acidic lubricant covering your lower body is just enough to allow you to slide forward through the hole, making you fall to the ground on the other side before your orgasm or the creature pistoning your ass slow down. You still shake on the ground, unable to focus on the world around you as the strong, new sensations overwhelm any rational thoughts.

After a moment your mind clears a little, and you're able to think through the rough assfuck that you're receiving. You look around, noting the sword and your shield nearby, and, most importantly, the mysterious creature latched to your hips. It looks more like an insect than an animal, being covered in white chitin-like surface. You try pulling it off, still feeling its acid burning your skin and making it more sensible without slowing down the thrusting into your poor hole, but the creature won't let go.

Before you can decide what to do, the creature speeds up again, announcing that something is approaching - and whatever it is, will be powerful. Your first instinct is grabbing the sword of St. Magdalena and pointing at the creature... but a second options comes to your mind.

Maybe you should leave it there? The creature is completely locked to you and, even though it's fucking your ass roughly - which is kinda enjoyable -, it would block anything else from going in there.

Whatever you decide to do, it has to be done quickly.