The Truth Hurts
No one really knows what these truths may be. To know it and speak, it is literally using the ability, itself. For this reason, even if you wanted to reveal it, you do so at the risk of ruining the one you share it with. Like a parasite, it would ravage any and all you speak it too, and only a small percentage will possess the composure to resist the downward spiral of madness. It's not that people can't discover these truths. All those who try to solve the puzzle will succeed on a long enough timeline. Therein lies the beauty of the eldritch tongue: It's a self keeping secret.
When these truths are spoken, anyone who hears it, and understands the words, will go pale in the face. They will feel sick as the realizations floods their thoughts. Their minds will obsess in the seconds that follow, repeating the phrase in their mind like a mantra. This is where the line between survival and certain death is crossed. If one cannot overcome this knowledge, compose themselves and move past it, they die. If one decides they cannot live in a world where these things are true, they die. They may go mad from the revelation, turning on their comrades with violent fury. They may have a sudden cardiac arrest, or in some cases, they may take matters into their own hands. This is not some power given at random like the prestige, nor is it a gift from higher forces. It is not a talent possessed by those with greater minds, it is merely the power behind speaking the truth.
Those who already know the truth spoken will be rattled, but otherwise unphased by the revelation. Those who possess severe mental instabilities are similarly unaffected. This means any one truth can used on a target once, and if they shrug it off, it can never be used again on them. The truth must be spoken clearly and with confidence. Any slurring or word barely heard by the target will leave something the target's mind can cling to, allowing them to ignore the words spoken or deny the truth entirely. There is something to be said about those particularly averse to candlelight. Some, such as Hennie Everstark, are so resilient to the development of candlelight that they can simply choose not to believe the truth spoken. Research into this effect is ongoing. It baffles those unaware of this peculiar outcome.