TENTACLES, TENTACLES, AND MORE TENTACLES
This is heavily based on an answer to a different question. Here's the link to the answer that is the original inspiration for this.
The Tayan puts tiny tendrils inside intelligent victims brains, that act as a dialysis machine keeping them alive, and feeding them nutrients.
The answer here is simple. You make the tiny tendrils mentioned above much larger and increase their numbers. Then, you make the tendrils do more than just act as dialysis machines; they also wrap around and thoroughly ensnare the victim.
The tentacles hold the victim off the walls of the stomach, ensuring that they have no leverage with which to free themselves regardless of strength. They also can't really attack the tentacles with teeth, fingernails, and such because the tentacles are constantly moving and can easily maintain their grip while avoiding the hands, feet, and mouth. And even if a victim gets lucky, the tentacles are smooth and covered with slippery saliva, making pulling hard enough to break them nearly impossible. And even if the victim gets lucky and manages to do enough damage to a tentacle to render it useless, there are plenty more to take its place.
Moreover, the victim would struggle to gather their strength because the tentacles would be coiled around their bodies, and could easily apply some light constriction that impairs breathing and leaves the victim constantly out of breath. The tentacles could also prevent victims from communicating with each other by gagging them or constraining their breathing.
And the tentacles could constantly churn the victim around, ensuring they are too disoriented to focus on any one thing. Even if the victim did free himself, he wouldn't be able to tell which direction he'd fall; a very paralyzing thing for people evolved from monkeys with good reason to fear death by falling from a tree.
Also, the victim would be unable to hide any tools, weapons, or other nasty surprises, as the tentacles will thoroughly search every inch of the body over and over, even checking under loose clothing. The victims won't be able to hide stuff on certain unspecified parts of themselves that they are reasonably certain won't be searched, because the tentacles are going to search those parts of you anyway, your dignity be damned.
Also, the slippery saliva-like substance mentioned earlier? That stuff could be the Tayan's digestive juices, and the tentacles will thoroughly coat the victim in the substance. If it were just a normal stomach, a victim might try to climb the stomach walls to get out of the pool of digestive juices near the bottom. But since the stomach has digestive juice-coated tendrils that grip the victims, the victim can't climb out of the digestive juices; all they can do is do their best to ignore the pain the acid causes.
The dialysis machine bit could be implemented by having each tentacle tipped with a stinger that can inject nutrients when the victim needs to be 'fed'. Of course, the stingers in question can inject more than just nutrients; they can also inject lovely, lovely drugs. Sedatives, muscle relaxants, drugs that screw with emotions... I'm just going to let your imagination think up the rest.
And all this also adds to just how unpleasant and terrifying the Tayan can be. The idea of being trapped in a wet sac, alone and slowly being digested is scary enough. The idea of being trapped in a wet sac, alone, and slowly being digested on top of being wrapped up in constricting tentacles that churn you around and occasionally inject you with god-only-knows-what is pure nightmare fuel.
Wait, what? I'm sorry, you want it to have a small flaw that could allow one human to escape? Okay then. Escape from this is nigh-impossible, but there is a pretty big emphasis on the nigh part of that.
The tentacles may be able to search every inch of the skin, but they can't search underneath the skin. Somebody could surgically implant a knife or other small cutting tool beneath their skin in the event that they were swallowed by a Tayan. If they are flexible enough, they could wiggle out of the tentacles, then use those moments of freedom to whip out the knife (ouch!). They could then use the knife to fend off the tentacles as they climb or cut their way to freedom.
