The acid tank :

The acid tank is a rather vicious trap.
It's a vat containing a special substance, irresistibly attracts masochists, who can't resist getting into it.
Once inside, ankles are cuffed. The mixture is unclear, but it is known to be particularly corrosive, melting the skin within minutes. It must also contain regenerating substances, because once the maso is raw, it stops dissolving.
A neurotoxin that increases pain is also believed to be present; an explorer once tried to dip a finger in it for a few seconds, and screamed in pain for hours, even though the skin had no time to dissolve and appeared intact.
There must also be neuro-stimulants, as victims are unable to shade into unconsciousness.
It's a bath of pure agony, where masochists squirm in indescribable pain for eternity… except for white maso, who soak in it as if they were in a hot pool.

For a long time, we thought there was no way out, until we understood that it was necessary to remain perfectly still and not make the slightest wave for 1 hour for the mechanism to release the victim.
This is impossible for almost all masochists, who will remain there for years, until they evolve to white level.
Their only hope is that a powerful entity will force them out, but most of them don't like acid and just walk away.

The only entity known to appreciate this substance is a tentacled monster, which is of no help to masochists, on the contrary.
It's perfectly adapted to acid and infests some of these vats. If a maso enters, it fucks her continuously, with an endurance that seems to have no limit, driving acid through every hole in the victim, who screams considerably louder than those in non-infested vats.