Purposive Constraint of the Monster
These kinds of monsters can be tricky in tabletop, since some see it as a "cheat" to have an unkillable, unstoppable monster, or feel that it takes away player agency. I think if the appropriate expectations are set, and the party is given a fair chance to fend for themselves, and the manner in which they figure out how to defend themselves requires ingenuity, then I think this could be a lot of fun and very much in-line with the philosophy of OSR. In terms of videogames, you could think of it like the Nemesis from Resident Evil; it is in effect more like a dungeon obstacle, or a horrific force of nature, in terms of how the party should interact with it.
It will never leave its lair, and in fact cannot leave its lair, even by magical means. It will hunt anything in its lair. It likes to toy with its prey, first revealing itself and attacking the party to get a measure of them, only to run off. It will stalk the party, re-emerging to strike whenever the party get too comfortable (if they are already overwhelmed, it will strike them at each room). It will usually toy with the party until they are just short of their goal, or too deep into the dungeon to easily escape, and then it will go for the kill. If it is not actively assaulting the party, it will allow other threats to face the party. However, if it is actively engaged with the party, it will casually deal with any other threats in its path.
It's first assault is generally a surprise attack, where it attempts to knock over, grapple, and pummel the most vulnerable member of the party. If the prey is strong, it will release a sticky fluid from its pores to better grapple. If this is insufficient, it will attempt to squeeze its limbs or whole body into any orifice of its prey. If there are no exposed orifices, or no seams in the protection of the prey, it will dribble its corrosive venom onto the protection until there is an opening.
Obviously this thing still needs a name. Also, I intentionally left out stats, with the idea that it should always be just a little bit above what the party would probably be able to defeat. They should have a small but reasonable chance of being able to escape if grappled, of being able to survive more than one surprise attack, and maybe even hit it (even if the damage doesn't mean much except to maybe stun it for a moment). This can be explained as the monster testing them, or toying with them, or even giving them a false sense of security that maybe they can beat it. Also, maybe I could do more with its appearance or abilities, or give it some explicit vulnerabilities? It clearly has some inspirations like the Nemesis from RE, the xenomorph from Alien, and spiders, but I think as-is it's fairly unique and I don't want to over-explain it in a way that takes away from that. Is this purposive constraint actually interesting? Does it meet this constraint?