Fair DC's for test of mental strength.

[Lucid]

In the near future my party (lvl 8-10) will be travelling to a demiplane library.
To enter they must solve a riddle and pass a test, here's what I have devised.

Before they can enter the portal they will have to decipher several inscriptions on two pillars, using either Arcana, History or Religion, if any of them can read Mulhorandi or Netherese they automatically succeed.

The first one is fairly simple: "When Horus-Re is at power the gate will open." DC 10 to decipher, DC 15 Religion check to know Horus-Re is a Sun deity.
I know riddles are tricky, and if necessary I'll allow intelligence checks for a hint, but I feel like it's straightforward enough.

The second inscription will give hints to the nature of the test.
DC 20 will give the correct translation which is: "Share of yourself and pass through. Resist and give up your self".
Passing DC 10 will wrongly translate as "Give up your self and pass through."

When they pass through the portal they'll find themselves isolated from each other in impenetrable darkness, with only the instinct to keep walking forward.

The test itself:
"You feel a presence in the darkness, pulling on your mind and soul, do you resist or accept?"
They have to make Wisdom save, if they accepted lower the DC by 5 and remove critical failure:
Critical failure - Fail, and you are blinded. "Your sacrifice is accepted, continue and do not stumble."
Critical success - You may pass freely. "Continue on your journey wise one."
DC 17 - Choose what to give, an important memory, spell or magic item(item is lost) "Share your knowledge and continue."
DC 12 - Choose what to lose, an important memory, spell or magic item "There must be an exchange to gain wisdom."
Fail - Lose an important memory, spell or magic item, DM decides "You may continue."
The reward for passing the test is an improved relation with the library keeper, who will be more amenable to help them.

I was wondering if the DC's I set for this are fair, or perhaps too low, and if the price for failure isn't too high.
Also if you think I should word things differently I'm happy to hear it, thanks!

[Unoriginal]

What are supposed to be "critical failure" and "critical success", in that context?

Also, the price for going through this door is pretty damn high no matter the success or failure. Spell-less characters will lose important memories or magic items, and that's a pretty damn big loss.

Other than that, I don't understand the purpose of the test, and why is it a Wisdom save

[Lucid]

What are supposed to be "critical failure" and "critical success", in that context?
A natural 1 or natural 20 respectively.

Also, the price for going through this door is pretty damn high no matter the success or failure. Spell-less characters will lose important memories or magic items, and that's a pretty damn big loss.

Other than that, I don't understand the purpose of the test, and why is it a Wisdom save The purpose is for it to be an admission price to the library, whose keeper is a lich that hoards knowledge and artifacts. It will serve as a semi-safe haven from the forces chasing them, where they can do research, learn new spells and I can dispense plot-critical information. The long-term gains will outweigh the loss.
I should have specified that in the op.

I wanted it to feel like there's a price to hidden knowledge, while still allowing admission. Instead of just refusing entry when failing which could split the party.

Success does not mean losing anything unless they choose to give up an item as it cannot be duplicated, a spell or memory will be shared, so they get to keep that. Also if they fail I don't think I will remove any magic item or spell, more likely a memory of something relating to their background which can come into play later.

As to it being a Wisdom save, it just feels most appropriate for this kind of test. I might extend it to any of the mental stats.

[Unoriginal]

Well, there is no crit success or fail in 5e for ability checks or saves, but if you want to add this, it's well within your rights as a DM.

Now, since this library is set up by a Lich who wants to get knowledge in exchange for admission, it doesn't make much sense that a) the Lich would set up an enigma instead of clearly stating the price and b) that they would leave it to a save DC to maybe work or not.

It'd make more sense, IMO, if the Lich simply wrote the conditions for entering in a way that can be understood, and then simply take what they want/get the shared knowledge as tribute.