Cyric has become a big part of the campaign I’m running, so I’ve decided to create a version of the Cyrinishad for 5e. I am wondering if I am being fair though. I’m trying to stay true to the lore of the book and have put the following in the description:

“Those affected by the Cyrinishad believe all written within the book to be true: Cyric is the one true deific power of the Material Plane, supreme above all other deities, and the only individual worth venerating. Priests and clerics of other deities immediately become clerics of Cyric (this does not change the PC's subclass). An individual continues to spread the “good word” of Cyric, trying to get as many converts as possible. Any religious leader, cleric or even Deity the reader encounters, they are compelled to convince them to also read the tome by any means necessary (trickery, intimidation, etc.). Being affected once by the Cyrinishad does not provide immunization from the book – one may be affected upon another reading. The only way to completely cure the reader of the affects of the Tome is by a wish spell or divine intervention (by any good deity).”

As you can see, there is no saving throw involved. Once you start reading, the affects happen (earlier in the description it also says once you start reading, you can not willingly stop). The question I have for my fellow DM’s is, is it fair to not have a saving throw to prevent the effects? FYI- my players are 15th level (I only mention that because I have been asked in the past to clarify the level).

It is ok to have things with guaranteed effects in the game, but I would consider a two-tier result for the effects. A high DC Wis save (25 or so), on a failure you're fully affected, but on a success, you suffer the long-term madness that you are always drawn to read the book again, and will not part with it willingly, even though you know it was bad. A low DC wis save at each rest could be the answer - pick a DC where there is a small chance of failure. Something akin to when someone is trying to kick doomscrolling on their phone - they're fine, until they stop doing something!

Well, if you're trying to be fair, then I'd say that you need to allow for at least one of two potential options:

• Would the PC(s) know the effects of reading the book before doing so?
• If no to the above, is there any way for them to avoid the effects once they start reading the book (i.e., a saving throw)?

You've already stated that the Cyrinishad does not allow for a saving throw, so the second option above is moot at the moment. So, for the sake of fairness, would the characters have any way of knowing what the Cyrinishad is and what it does before one of them actually starts reading it? Are you giving them a Intelligence (Religion) or Intelligence (Arcana) check beforehand to realize the potential dangers? If there's no way of knowing about the potential danger and no way to avoid it once you start reading, then it could be argued that it isn't necessarily fair... Not that Cyric particularly cares about fairness, but we do for the sake of your question.

Now, having said that, you also stated that the characters are 15th-level and a wish spell can undo the effects of the Cyrinishad. Characters don't have access to 9th-level spells yet, so they wouldn't have wish available for at least a couple more levels. This fact also means that they have no quick way to restore their characters back to normal. If you're group is fine with roleplaying that change, then go with it. If your group is going to feel like they're losing agency for something potentially unavoidable, then you might need to change things slightly to give them at least a chance.

Similar to with Thoruk said, I would maybe give a reader a one time Wisdom (or Charisma) saving throw as they begin looking at the book to realize somethings not right and stop doing so. I'd probably aim at a DC 21, which would make it very difficult for most normal folk but possible for a heroic character to pull away at the last minute. They might not learn anything about the specific effects beyond that the book had an insideous effect tug at their mind. If they choose to continue reading to learn more, then they also choose to accept the effects and Cyric gains a new follower. Goes agains the original concept of the Cyrinishad a bit, but it allows the PC(s) a more fair chance. 

I’ve added this to the text as well, 

“Once someone starts to read the tome they must make an intellegence Saving through (DC 25). On a successful save they are able to read the text in their native language without suffering a form of madness. On a failed save the text becomes gibberish for 24 hours to the reader and the player suffers a form of madness (roll 1d6 to determine what kind 1-2 short term, 3-4 long term, 5-6 indefinite).  A calm emotions spell can suppress the effects of madness for 1d4 hours, while a lesser restoration spell can rid a character of a short-term or long-term madness. Remove curse or dispel evil and good are also effective. A greater restoration spell or more powerful magic is required to rid a character of indefinite madness. In addition to the madness, the player has an insatiable need to read the tome until they can understand it. The save must be repeated each time the tome is read, regardless if you have saved before or not.“

to make it a bit more worthwhile, I also added some high level spells to learn and considering a +1 to charisma for reading it (similar to the Tome of knowledge).